A military term. Capitulation refers to surrendering or giving up. In the stock market, capitulation is associated with "giving up" any previous gains in stock price as investors sell equities in an effort to get out of the market and into less risky investments. True capitulation involves extremely high volume and sharp declines. It usually is indicated by panic selling.
After capitulation selling, it is thought that there are great bargains to be had. The belief is that everyone who wants to get out of a stock, for any reason (including forced selling due to margin calls), has sold. The price should then, theoretically, reverse or bounce off the lows. In other words, some investors believe that true capitulation is the sign of a bottom.