A put option whose strike price, at a given point in time before or at expiration, is equal to the underlying price. As such, the holder will be indifferent as to exercise the option or not. That means, whether the option the option is exercised or not, the holder nets no payout at all. For example, suppose the stock of XYZ is trading today at $50, and an investor expects the stock price would edge down to $40 in the near future before expiration. Therefore, the investor may buy an XYZ 50 put based on that view. However, if at expiration, the stock price closes at exactly $50, then the put option is said to expire “at the money” or worthless.