Assignment
The obligation placed on an option seller to fulfill the terms of the contract when the buyer exercises.
Explanation
When a short option holder is assigned, they must buy (put) or sell (call) the underlying asset at the strike price. Assignment can happen at any time for American-style options, though it most commonly occurs at expiration or around ex-dividend dates for calls.
Example
You sold a 100-strike call on AAPL. The buyer exercises, and you are assigned: you must deliver 100 shares of AAPL at $100 per share.