A holding of (normally high) cards in the same suit, one ranking next above and the other next below a card held by an opponent, eg A-Q or K-J. A holding that is not strictly a tenace before play starts may become one during the course of play. For example. A-J-x opposite x-x-x becomes a tenace if the king is led.
If your side leads a suit in which you have a tenace, you want to lead towards rather than away from the tenace. If the opponents lead the suit, you would prefer a lead up to rather than through the tenace.