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All the option contracts of the same class on the same underlying security with the same strike price and expiration date that are traded on an exchange. For example, all call options on Stock X with strike price Y expiring on the last Friday in March make up one option series in the exchange on which they are traded. An option series is also simply called a series.
All option contracts on a stock that have an identical expiration date and strike price. An example of a series is March options on IBM with a $130 strike price. Also called series.