Max Pain
The strike price at which option holders collectively would suffer the maximum financial loss at expiration.
Explanation
Max pain theory suggests that the underlying price tends to gravitate toward the strike price where the most options (by dollar value) expire worthless. While controversial, many traders monitor max pain levels as a potential support/resistance indicator, especially during expiration week.
Example
If max pain for AAPL at Friday expiration is $150, the theory suggests AAPL will gravitate toward $150 as market makers hedge and pin the stock.