Capitulation is the emotional end of a selloff. volume spikes, prices gap down, and fud is everywhere. It is only identifiable with certainty afterward; plenty of capitulation-looking days are followed by more decline.
Traders watch for a high-volume reversal candle with a long lower wick as one possible signal.
Example: after a 45% decline, a stock falls 15% in one session on five times average volume and then closes near its high. That session later proves to be the low.
Related: dead-cat-bounce, volume, wick, fud