Option expiry is the date and time at which an option contract comes to an end. After it, the right to exercise lapses. The date is fixed in the contract as its expiration date. At that moment an option has no remaining time value and is worth exactly its intrinsic value: one that is in the money is exercised, normally automatically, and one that is out of the money expires worthless.
Listed options expire on a regular calendar. Standard monthly US equity options expire on the third Friday of the month, and exchanges also list weekly and, for major indices, daily expiries. A European option can be exercised only at expiry, while an American option can be exercised at any time up to it. As expiry nears, time decay, measured by theta, speeds up, and for options near the money gamma becomes very high, so small moves in the underlying cause large swings in value.