Definition of semi-bluff
On the one hand, there is the hard bluff, i.e. betting with a hand that has no chance of being the best, and on the other hand, there is the value bet because you very often have the best hand. Between the two, there is the semi-bluff. The semi-bluff is a play made only on the flop or the turn. To call a semi-bluff, there must be cards left to come. Generally, a player who bets or raises in semi-bluff does so because on the one hand, he thinks he can get his opponent to fold often enough, and on the other, because if he is called, he has a draw to save him.
A classic example of a semi-bluff
A player opens on the button with KJ in spades. The player in SB position calls the raise. The player in BB position folds. The flop comes 2-5-7 with two spades. SB checks and the player in position bets the flop. This bet will be considered a semi-bluff. It is not a straight bluff with a hand like T4. With KJ, there is a flush draw and two overcards. The player is said to have made a semi-bluff bet.