A One Turn Kill (OTK) is achieved when a player wins a Duel in the space of a single turn, as soon as said player has the appropriate cards in hand and has control over the duel. This is similar to a First Turn Kill, but the latter is pulled off during the player's first turn, while an OTK can be accomplished at any point of the duel (meaning that all FTKs are OTKs, but not all OTKs are FTKs).
Kaiba: Ha! In three turns, I shall summon my Blue Eyes White Dragon and decimate your remaining life points!
Yugi: Perhaps, if victory were not within my grasp this turn. Prepare for an OTK!