From AMS Glossary
A body of water with a common formation history, for example, convection caused by surface cooling, having its origin in a particular region of the ocean.
Water masses are identified by their temperature, salinity, and other properties such as nutrients or oxygen content. They have exclusive occupation of an oceanic region only in their formation region; elsewhere they share the ocean with other water masses with which they mix. Just as air masses in the atmosphere, water masses are physical entities with a measurable volume.