VPN -- "virtual private network." My definition off the top of my head: an encrypted channel from a user's location (say, in a BA lounge) to some trusted location (say, an ISP or office). Used to prevent the location (which you might not trust particularly!) from intercepting or otherwise tampering with Internet communications to/from user.

Webopedia's definition: http://www.webopedia.com/TERM/V/VPN.html