The number of contracts open in a particular option asset, which is updated after market (regular session) close every day. If open interest is 600, for instance, that means there are 600 two-party contracts (buyer and seller). Open interest goes up when the parties involved are both initiating a new positions; it goes down if both parties are closing an existing position (one party closing a long position while the other closes a short position). At expiration, open contracts in the money are executed, and those out-of-the-money expire worthless.
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