That is correct for a qualifying flight but does not define a sector as questioned by the OP
Would be good to clarify this for the OP. What I am reading from the EK website, is that you need 25 qualifying flights not 25 sectors on qualifying flights to retain Silver.
http://www.emirates.com/english/help...latinum-status
Once you have reached a new tier you have a fixed 12-month period in which to earn the Tier Miles you need to keep your status. During this time, you would need to save 25,000 Tier Miles or take 25 qualifying flights to remain in Silver, 50,000 Tier Miles or 50 qualifying flights to remain in Gold and 150,000 Tier Miles to remain in Platinum.