Pin Risk
The risk that the underlying price closes very near a strike price at expiration, creating uncertainty about assignment.
Explanation
When a stock pins near a short strike at expiration, you may not know whether you will be assigned until after market close. This creates overnight risk and potential unwanted stock positions. Pin risk is especially problematic for short straddles and short strangles near expiration.
Example
You sold the 150 straddle on AAPL. The stock closes at $150.02 on expiration Friday. You may or may not be assigned on the call, creating uncertainty.