Range
All the hands someone could hold in a spot. You play against ranges, not exact cards.
Say you're in the big blind and call a raise from the cutoff. The flop comes Kh 9c 4d and the cutoff bets. You don't know their two cards, but you know their range — the hands they'd open from the cutoff and keep betting here. Kings and queens, ace-king, ace-queen, king-jack for top pair, a few suited connectors that caught a piece, and a pile of ace-high hands still bluffing. Your call, raise, or fold answers to that whole spread, not to a guess at one hand.
Playing the range instead of the hand is what keeps you honest over the long run. A single pot can go badly even when you played it right: you call a bluff and run into the one value hand in twenty, or you fold the best hand because most of the range beats you. Ranges also move street by street. A card that pairs the board or fills a flush helps part of the range and kills the rest, and sorting out which part is how you find the profitable bluff or the profitable fold.
The beginner mistake is shrinking a range to one hand — "he has ace-king" — instead of weighing the dozen hands that fit the action. That leaves you either too scared, folding to a range that's mostly weaker, or too stubborn, calling off against one that's mostly stronger. Ask what a typical hand in this spot looks like, not what the scariest one is.