The nuts
The single best possible five-card hand on the exact board in front of you, a property of the board itself rather than of any hand you hold.
Working out the nuts starts before you look at your own cards: what does the board already make, and which two hole cards push furthest past that? The answer is not fixed for the whole hand. A fourth card of a suit can turn a made set into second-best behind a flush, and a paired turn card can introduce full houses and quads that were not live a moment before, so each street deserves its own read rather than inheriting the flop's answer.