Range
The full set of hands a player could hold in a spot, each weighted by how likely it is, rather than a single guessed hand.
Thinking in ranges rather than single hands is the foundation of modern poker strategy. A player's range narrows with every action: a raise from early position tells you more than the same raise from the button, where far more hands are profitable to open, and each further bet, check or fold narrows it again. Reading ranges well means weighing an opponent's whole distribution of hands, not just the one you fear most or the one you hope they have.