AA = American Airlines or AAdvantage (it’s contextual)
AAR = Airport Acceptance Rate (permitted rate of # of flights landing per hour)
AATMS = Advanced Air Traffic Management System
AC = Admirals Club lounges; sometimes, Aircraft (aircraft also a/c)
ACARS = Aircraft Communications Adressing and Reporting System
ACAS = Airborne Collision Avoidance System
ACMI = Aircraft lease with Aircraft, Crew, Maintenance and Insurance
AD = Airworthiness Directive, an FAA rule regarding specific aircraft safety issues
ADC = Air Data Computer
ADC = Code used by AA to indicate "traveling companion - remove 'stickers' from primary traveler"
ADS = Automatic Dependant Surveillance, advanced GPS based navigation system
ADT = Adult Passenger
AFS = American Flagship Service (2-class or 3-class nonstop transcon service)
Aft = rear
AFTN = Aeronautical Fixed Telecommunication Network
AHRS = Attitude and Heading Reference Systems 
AONE = oneworld explorer ticket in First
ARINC - Aeronautical Radio, Inc.
ARTC = Air Route Traffic Control Center
ASM = Available Seat Mile
ASR = Advance Seat Reservation / Airport Surveillance Radar
ATC = Air Traffic Control
ATCT = Airport Traffic Control Tower
ATCSCC = Air Traffic Control System Command Center
ATI = Anti-Trust Immunity
ATO = Airport Ticket Office
ATPCO = Airline Tariff Publishing Company)
AVOD = Audio Video On Demand (form of IFE)
BC = Business Class
BIS = "Butt In Seat" miles (EQM, gained by actual flying)
Bistro Bag = coach bag lunches retrieved from carts on boarding (obsolete)
BOB = Buy on Board; see FFS
BOGOF = Buy One Get One Free
Booking / Fare Classes = see this Wiki page
Booking Class "7" = Upgrade to next class of service from highly discounted coach 
Booking Class "8" = Upgrade to First from Business 
Booking Class "9" = Upgrade to next class of service from discounted coach 
BP = Boarding Pass
BT = Bulk Ticket
Bulkhead = "wall" separating fuselage sections
C (or D, or J) = Business Class cabin or fare class (contextual)
C01 – C17 = Child passenger and age code
CAS = Crew Alerting System (in aircraft cockpit)
CASM = Cost per Available Seat Mile
CB = Cumulonimbus clouds
CFC = Confirmed Flight Change
Challenge, the = a way of attaining Gold or Platinum status via a "shortcut" - see the Wiki or the thread dedicated to Challenges
Cheatline = the colored line often extending down the fuselage or an aircraft
"x" Check, where "x" is A, B, C or PS = periodic maintenance - see here for AA 
CG = Center of Gravity (see Weight and Balance)
Circle Pacific = a discounted oneworld product allowing that routing
City Pairs = airports considered the same for fare construction purposes
CIR = Circle Pacific, class precedes, MPM follows, e.g. DCIR22
Co-terminals = Airports considered the same for standby and similar procedures
ConciergeKey (or CK) = Premium Service for the most valued AA customers
CONUS = Continental United States (contiguous 48 states)
COS = Class Of Service (referring to cabin service class)
Counter Upgrade = upgrade offered for sale at the ticket / check-in counter
CPM = Cost Per Mile
CSA = Customer Service Agent
CSR = Customer Service Representative (Corporate Social Responsibility and others as well)
CX = Cathay Pacific (secondary: cancellation)
Direct (flight) = flies with one or more stops, possibly even change of aircraft
DME = Distance Measuring Equipment
DOMUPG = Domestic Upgrade
DONE = oneworld explorer ticket in Business
DOT - (US) Department of Transportation (includes the FAA)
DSR = Passenger desiring to purchase up class of service
ECAM = Electronic Centralised Aircraft Monitor, same as EICAS, preferred Airbus version
EFIS = Electronic Flight Instrument System
EICAS = Engine Indicating and Crew Alerting System (heart of the "glass cockpit") same as ECAM
Emerald = oneworld equivalent of AAdvantage Executive Platinum
EP = Executive Platinum
EQM = Elite Qualifying Mile (a milepost used to determine status; "butt in seat" mile)
EQP = Elite Qualifying Point (ditto)
EQS = Elite Qualifying Segment
ETA = Estimated Time of Arrival
ETD = Estimated Time of Departure
ETER = Estimated Time En Route
EXP = AAdvantage Executive Platinum (chiefly on FlyerTalk; Also EP) 
EXP Line = A dedicated help desk for Executive Platinum flyers
eUPG = Electronic Upgrade
eVIP = electronic VIP international Systemwide Upgrade "certificate" or SWU
   usable for up to three segments in one direction, on most fares
F (or FC, A) = First Class cabin or fare class (A is normally discounted First)
FA = Flight Attendant
FAA = Federal Aviation Administration, part of US DOT
FAM = Federal Air Marshall
FC = First Class
FCC = Usually, London Heathrow Flight Connections Centre (on FT)
FEBO = (unofficial) Front Even, Back Odd (meal taking on many flights)
FFS = Food for Sale program in American Airlines domestic Coach 
FL = Flagship Lounge (or Flagship, contextual)
FO = First Officer (“copilot” - but see PF and PNF)
FOP = Form Of Payment
FQTR = Frequent Traveller Redemption (on BA)
FQTV = Frequent Traveller (on BA)
FS = AA's premium transcontinental Flagship Service
FT = FlyerTalk
GA = Gate Agent
Gate Lice = people with low boarding priority cluttering up the gate at boarding time
GDP = Ground Delay Program
GDS = Global Distribution System
GLD = AAdvantage Gold
GMT = (Greenwich Meridian Time) Time at Greenwich Meridian, used for some airline and military operations 
   (see UTC, Zulu Time)
Gold line = a dedicated customer service line (not separate desk) for AA Gold 
GPS = Global Positioning System (satellite based navigation system)
GS = Ground Stop
GV x = Passengers must travel together, x = number of pax
Heavy = Larger aircraft, such as 767, 777, 747, A330 - 340
HFS = Hawaiian Flagship Service
HSR = High Speed Rail
HUD = Head Up Display
IAS = Indicated Air Speed (usually in knots)
IDB = Involuntarily Denied Boarding (involuntary bump)
IFE = In Flight Entertainment
IFR = Instrument Flight Rules
IFS = International Flagship Service (to/from Europe, Asia, India, and "Deep" South America - Argentina, Brazil, Uruguay and Chile)
IFSD - In Flight Shut Down  (usually relating to an engine)
ILS = Instrument Landing System
IROPS/ IRROPS = Irregular Operations (OSO on AA - could be weather, mechanical...)
ISIC = International Student Identity Card (12 or older currently enrolled as a full-time   
  degree-seeking student at an accredited institution)
ITA = Anti-Trust Immunity
ITIC = International Teacher Identity Card (Teaching > 18 hrs / wk at an accredited institution, min 1 yr employment.
IYTC = International Youth Travel Card (age 12 - 25)
JCB = (Adult) Contract Bulk passenger
Jim Wilson - Human remains traveling in the cargo hold ("Jim Wilson Air Tray" is an air casket)
Kiosk = Self-Service Check-in Machine
Knot = unit of speed, one nautical mile per hour, kt or kn (link)
KUP = Low cost fare booking into First seat (BA F status credit!)
LAADR = Low Altitude Arrival/Departure Routing
LAHSO = Land and Hold Short Operations.
LEO = Law Enforcement Officer 
LFBU = Load Factor Based Upgrades sold at self-service check in kiosks
LFS = Latin Flagship Service (South America short-haul: Colombia, Venezuela, Ecuador, and Peru)
LIFO = Last In, First Out
LOC = Localizer (component of ILS)
LONE = oneworld explorer ticket in Coach
LRTC = (unofficial) Less Room Throughout Coach, followed MRTC
MCLA = Mexico, Caribbean and Latin America
MCO = Miscellaneous Charges Order (a kind of voucher)
MEL = Minimum Equipment List (list of equipment allowed inoperative not affecting flight safety)
METAR = MÉTéorologique Aviation Régulière (Aviation Routine Weather Report)
midcon = flight that flies mid-continent, e.g. ORD-JFK or DFW-LAX
MIL = Military Passenger
MPM = Maximum Permitted Mileage
MR = Mileage Run (trip planned primarily to earn miles, at a cheap rate)
MRTC = More Room Throughout Coach (obsolete)
MTEU = Miles Towards Elite Upgrades (counter runs from March 1 to February 28/29; elites earn 4 500 mile "sticker" upgrades every 10,000 EQ miles)
MTOW = (aircraft) Maximum Take Off Weight
MX = Mexicana (secondary: Mechanical)
NGBC = Next Generation Business Class - new angled flat seats in 763s, coming to 777s in 2008
NGSA = Next Generation Single Aisle
Nonrev = Nonrevenue (e.g. pass) flyer
Nonstop = flight that flies from origin to destination with no stops
NOTAM = NOTices to AirMen (voice or teletype information of interest to pilots)
NRP = North American Route Program
NRSA / Non-Rev = Non-Revenue Space Available flyer (e.g. NRSA Lounge at DFW)
O&D = Origin and Destination
OAG = Official Airline Guide
OH, OHB = OverHead, OH Bin
OLCI = On Line Check In
OP = Original Poster (on FlyerTalk)
Op Up = Operational Upgrade
ORC = Original Routing Credit
OSO = Off-Schedule Operations (see IRROPS)
OTS = Out Of Service
OW = One Way (trip) - OR - oneworld (Alliance) on FlyerTalk
OWE = oneworld Explorer (RTW) fare - prefixed with cabin class code, followed by # of continents - e.g. DONE4
PAWOB = Passenger Without Bags 
PDB = Pre-Departure Beverage
PED = Personal Entertainment Device
PF = Pilot Flying
PFD = Primary Flight Display or Personal Flotation Device (contextual)
PIC = Pilot In Command (SIC = Second In Command)
PIL = Passenger Item List (manifest)
PIREPS = Pilot Reports
Pitch = aircraft motion "up or down"
Plat = AAdvantage Platinum
Plat (or PLT) Lite = one who achieved Platinum status via a Challenge
Platinum line = a dedicated customer service line (not separate desk) for AA Platinum 
PLT = AAdvantage Platinum
PMA = Parts Manufacturer Approval 
PNF = Pilot Not Flying
PNR = Passenger Name Record, the "airline record" of your booking, usually 5 or 6 characters and airline specific
Port = Aircraft's left side as you face forward
PPR = "paper," an edit used to assure an op up doesn't automatically draw upgrade instruments, 
   usually used by the GA 
PMA = Parts Manufacturer Approval, an FAA process for aircraft parts design and manufacturing
PQ = Price Quote
PTA = Prepaid Ticket Advice 
PYOB = Print Your Own Boarding Pass
RAPCON = Radar Approach Control Facility
RASM = Revenue per Available Seat Mile
RDM = Redeemable Mile (any mile that can be redeemed for awards, etc.) 
RI = Passenger with interrupted OSO travel
RJ = Regional Jet OR Royal Jordanian (contextual)
Roger = acknowledgment used in radio communications (e.g. with control tower)
Roll = aircraft motion about the longitudinal axis
RON = Remain Over Night
ROW = Roswell Industrial Air Center, NM "the boneyard" where old aircraft go - formerly Walker Air Force Base  
RPM = Revenue Passenger Mile
RT = Round Trip
RTW = Round The World (ticket, fare or desk arranging same)
Ruby = oneworld equivalent of AAdvantage Gold
Rule 240 = Airline's domestic reaccommodation policy (AA: US, Canada, Puerto Rico, USVI) 
Rule 80 = Airline's international reaccommodation policy
RV = Revenue Passenger
RVR = Runway Visibility Reporting 
RWY = Runway
SAC = Special Assistance Coordinator 
Sapphire = oneworld equivalent of AAdvantage Platinum
SATA = Student Air Travel Association
SDFC = Same Day Flight Change
SDR = Special Drawing Rights (international currency code XDR) 
SELCAL = Selective-Calling system (usually signal for messaging from operator to aircraft)
Short Check (bag) = Checking bag to an intermediate, not the final, destination
SIC = Seat In Coach
SIF = Selective Identification Feature (aircraft-based transponder signal)
SIGMET = SIGnificant METeorological Information
SPML = Special Meal (not available on all flights)
SPCCR = Self-Policing Community Closure Rule, used by some on this forum for proposing ignoring a thread
SRC = Senior Citizen Passenger
SSSS - Selected for Secondary Security Screening 
   (on USA BP - TSA Security will give you an extra lookover))
STC = Supplemental Type Certificate (FAA approval doc for aircraft modifications, changes or additions)
STD = Scheduled Time of Departure
STMP = Special Traffic Management Program
STPC = Stopover Paid By Carrier
Starboard = Aircraft's right side as you face forward
Sticker = 500 mile upgrade instrument (used to be actual stickers)
SUD = Boeing 747 Stretched Upper Deck (2nd or upper deck)
SVRWX = Severe Weather
SWAP = Severe Weather Avoidance Plan
SWU = Systemwide Upgrade (electronic international upgrade "certificate" on AA, often "eVIP")
TA = Ticket Agent (Travel Agent, sometimes)
TAS = True Air Speed (usually in knots)
TATL = Transatlantic; sometimes transat
TCAS = Traffic Alert and Collision Avoidance System
TCP = Total and Complete Party (Traveling together)
TPAC / Transpac = Transpacific; often transpac
TRACON = Terminal Radar Control Facility
transcon = flight crossing US coast-to-coast (Atlantic & Pacific coasts)
tstorm = thunderstorm
TTL = Ticketing Time Limit
TYP = Thank You Points
UG = UpGrade / UPG
UM or UMNR = Unaccompanied Minor (14 or under, optional 16 or 17)
US = Unserviceable / out of service
UTC = Universal Coordinated Time (see GMT, Zulu Time)
VCR = Virtual Coupon Record
VDB = Voluntary Denied Boarding (voluntary bump)
VFR = Visual Flight Rules
VIP = Systemwide Upgrade  (See eVIP; also VIPOW, obs.)
VOR = VHF (Very High Frequency) Omnidirectional Radio Range
VORTAC = (Combined) VOR and TACAN Navigational Facility
Weight and Balance = calculations affecting aircraft flying characteristics based on maximum and distributed wieght
WiFi Lice = non-lounge users who hang out near lounges to use WiFi signals
Wilco = "Will Comply," occasionally used in radio communications (e.g. with control tower)
WL = Wait List
WX = Weather
Y = Economy cabin or full fare Economy fare (contextual)
Yaw = aircraft motion "side to side"
YM = Yield Management
YMMV = Your Mileage May Vary
YRT = One year ticket
YUP / KUP = Lower priced domestic fares booking into next class of service cabin
Z following 24 hour time, or Zulu Time = see GMT, UTC

Abbreviations frequently encountered on the Internet, e.g. FWIW = For What It’s Worth, IME = In My Experience, IMO / IMHO = In My (Humble) etc.Opinion

Mainline aircraft designations and number in fleet (#616, down from 663 EOY 2007, as of Feb 2009) <link>:

AB6 = 23 (previously 34) Airbus 300-605R two-class wide body
737* = 77 Boeing 737-823 two class
757* = 124 Boeing 757-223, two class (757-121s ex-TWA gone)
757 L - 757 (18 to be in subfleet) with NGBC / Next Generation Business Class seating, 12 seats per cabin instead of usual 16
762 = 15 Boeing 767-223ER, three class transcontinental service
763* = 58 Boeing 767-323ER, two class cabins in domestic and international service (all with new near-flat J seats)
777 = 47 Boeing 777-223ER/IGW, 3-class cabins (all Flagship Suites in F, 45 with NGBC near flat seats in J)
S80 / MD80, M83 = 272 (recently 300) McDonnell Douglas MD-80 series, essentially stretched DC-9s (AKA "Mad Dog")
(revised Dec 2009)

* received / receiving Aviation Partners Boeing blended winglets

Airlines of the oneworld alliance

AA = American Airlines
AY = Finnair 
BA = British Airways
CX = Cathay Pacific Airways (Hong Kong, China)
IB = IBERIA (Spain)
IT = Kingfisher Airlines (India-based, pending, mid- or late 2011)
JL = Japan Airlines, including JAL associates JALways, Japan Asia Airways, JAL Express, J-AIR and Japan Transocean Air
KA = Dragonair (China) as of 1 Nov 2007
LA = LAN airlines (Chile and Perú) now includes 4M = LAN Argentina and XL = LAN Ecuador
MA = Malév Hungarian 
MX = Mexicana (and Click Mexicana) joined in late 2009
RJ = Royal Jordanian
QF = QANTAS (Australia)
S7 = S7 Airlines, Russia-based, expected to join late 2010)

(N.B. Aer Lingus has departed oneworld; SN / Brussels Airlines, Swiss and Turkish never were members.)