An Olympic-size swimming pool is the type of swimming pool

Swimming pool

A swimming pool, swimming bath, wading pool, or simply a pool, is a container filled with water intended for swimming or water-based recreation. There are many standard sizes; the largest is the Olympic-size swimming pool...

 used in the Olympic Games

Olympic Games

The Olympic Games is a major international event featuring summer and winter sports, in which thousands of athletes participate in a variety of competitions. The Olympic Games have come to be regarded as the world’s foremost sports competition where more than 200 nations participate...

, where the race course is 50 meters in length. This is typically referred to as "long course", delineating it from "short course

Short course

In Swimming, the term Short Course is used to identify a pool that is 25 metres in length. The term is also often included in meet names when conducted in a short course pool...

" which applies to competitions in pools that are either 25 meters or 25 yards in length.

The long course size is commonly used as a casual measure of volume.

Informally, "olympic-sized" is used as an adjective meaning "much larger than normal," and not necessarily, "meeting official Olympic specifications." Many hotels, homes and other venues describe their pools as "olympic-size" simply because they are at least 50% larger than typical.