The crux is the most difficult sequence of a climb — the move (or short series of moves) that defines the route's grade. Most climbs have a single distinct crux; harder climbs sometimes have multiple cruxes separated by rest moves. Identifying the crux before attempting a route lets climbers conserve energy and prepare mentally for the key sequence.
Cruxes vary in character. A "powerful crux" is short but extremely hard — a single move at near-maximum strength. A "techy crux" demands precise body positioning rather than raw power. A "thuggy crux" is sustained — many hard moves back-to-back without rest. A "balance crux" demands subtle weight transfer and footwork. Different climbers find different cruxes hard depending on their strengths.
In route guidebooks, the crux is usually noted ("crux: third bolt"). On an indoor route, the crux is often where setters built the most awkward or reachy sequence. Climbers identify the crux during a "preview" — examining the climb from below, working out where the difficulty lies — and approach it with the most chalk, focus, and commitment.