1) The date on which an option, right or warrant expires and becomes worthless if not exercised. For stock options, this is the third Saturday of the month in which the contract expires, or the third Thursday of the month if the third Friday is a holiday. Options on equity indexes can vary, e.g., SPX expires on the third Friday, but options on the VIX expires on the third Thursday.
2) The date on which a futures contract expires; if the futures contract is not offset prior to expiration, delivery (either by way of cash settlement or physical settlement depending on contract specifications) of the commodity or financial instrument ensues. Expiration dates vary by the particular futures contract.