An onsight (sometimes "on-sight") is climbing a route cleanly on the first try with no prior information. The climber may not have watched anyone else climb it, read beta, seen photos of the holds, or studied a topo. They walk up, tie in, and figure out the moves as they go.
Onsighting is climbing's purest style. It demands not just the physical strength to do the moves, but the mental composure to read the climb under stress, find the right holds, and execute decisions on the fly without fallback. Most climbers can redpoint substantially harder than they can onsight — the gap is typically 1–3 grades.
Onsight is contrasted with flash (clean first attempt but with beta) and redpoint (clean send after prior attempts). Climbing competitions in the lead format are pure onsight events: climbers are isolated from the route until their attempt, see it for the first time on the wall, and have one go to climb as high as possible. Onsight grades are tracked separately in climbing CV tradition because of how much harder they are than redpoint grades.