No term in fantasy football is more popular than "sleeper," as success is predicated on drafting players whose production far exceeds their asking price.
To be in consideration here, a player's average draft position at My Fantasy League must fall outside of weekly starter territory in 12-team leagues. The farther outside that top 12 (or top 24 at running back, or top 36 at wide receiver) the player falls, the bigger the sleeper potential.
If you can snag a player that late who goes on to have a big year, well...that's how you win fantasy leagues.