Series

A series is the most specific way of identifying an options contract. It narrows things down from the general to the exact: first you have an underlying stock or index (say, a particular company's shares), then a class (all the call options on that stock, or all the put options on that stock), and finally a series, which is one exact combination of expiration date and strike price within that class. Every contract in a series is economically identical — same underlying, same right to buy or sell, same date it expires, and same price at which it can be exercised.

In practice, when someone says "the series," they mean something like "the 150 calls expiring the third Friday of next month" — every single one of those contracts, no matter who bought or sold them, belongs to that one series. If you instead pick a different strike price, say 155, or a different expiration date, you are now talking about a different series, even though it is still the same class and the same underlying stock.

The nuance that trips people up is the layering of terms: underlying, class, and series sound similar but describe different levels of grouping. Class is broad (all calls, or all puts, on one stock, across every strike and expiration). Series is narrow (one specific strike and one specific expiration within that class). People sometimes use "series" loosely to mean any options contract, but strictly it refers to the whole group of identical contracts sharing those two defining features, not a single contract someone holds.

This matters practically because exercise, assignment, and settlement all happen at the series level — when an option is exercised, it's contracts within that specific series that get matched up, not contracts from a different strike or expiration.