Expiration Date
An option's expiration date is the last day the contract is valid; after it, the option can no longer be exercised and either settles or expires worthless.
Standard monthly US equity options expire on the third Friday of the month. Many stocks, ETFs and indices also have weekly options, and some major index products have options expiring every trading day. Options can be listed months or even more than two years out; long-dated ones are called LEAPS.
Most US stock options are American style, meaning they can be exercised any time before expiration. Many index options are European style and can only be exercised at expiration; they also settle in cash rather than shares.
Time is a key part of an option's value. All else equal, an option with more time left is worth more because the underlying has more chance to move. This time value shrinks as expiration approaches, a process called time decay, measured by the Greek theta, and it accelerates in the final weeks.
Example: an in-the-money option at expiration is typically exercised automatically by the clearing house if it is at least $0.01 in the money.