Definition
A bluff is a bet or raise made with a hand that is unlikely to win at showdown, with the goal of making better hands fold. Bluffing is fundamental to poker because without it, strong players become predictable — opponents would simply fold when facing bets and only call with stronger hands.
Effective bluffing requires understanding the opponent's likely range, the board texture, the size of the bet, and the story your action tells. "Semi-bluffs" are generally more powerful than pure bluffs — these are bets made with drawing hands that can win two ways: if the opponent folds now, or if the draw completes.
Bluffing frequency should be calibrated to pot odds and opponent tendencies. A bet that gives your opponent 2:1 pot odds needs to succeed 33% of the time to break even. The best bluffs have multiple paths to winning and are made in positions where the story is believable.
Example
You raise from the Button with 5♣4♣, the Big Blind calls. The flop is A♠K♣3♦ — a board that connects strongly with your preflop raising range (AK, AQ, KQ). You bet 70% pot as a continuation bet bluff; the Big Blind folds. The board helped your bluff land even though it missed your actual hand.