The tendency of a stock's price to close near the strike price of heavily traded options (in the same stock) as the expiration date nears.

This doesn't always happen, but it often does when there is significant open interest. For example, if a stock is trading near $50 and there is heavy trading in both puts and calls at this strike price, there is a tendency for the stock price to be "pinned" at $50 as traders unwind their positions at expiration.