A semi-bluff is a bet or a raise made with a hand that, while not currently the best, has a decent chance of getting there. If your opponents fold to your aggressive bet, fantastic; if they don't, you still have a decent shot of drawing to a winning hand. Semi-bluffing is most often used to get a 'free card' on the turn or river by showing aggression early in order to be checked to later on in the hand.
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