Fold Equity
The portion of an aggressive line's value that comes from your opponent folding.
When you raise or shove with a hand that has decent equity when called, your play has two sources of profit: the times you win at showdown, and the times your opponent folds. Fold equity is the second component.
Semibluffs are the textbook example. A flush draw has ~35% equity if called, but if your opponent folds 40% of the time to a raise, the raise becomes profitable even when your raw equity is below the calling price.
Calculate fold equity quickly: estimate (a) how often opponent folds, and (b) the size of the pot you steal. Add this to your showdown EV when called. Most "should I bluff?" questions answer themselves once you do this math honestly.