A sea or an ocean, according to Defant (1929), is composed of wat er masses which are more or less homogeneous volumes of water characterized by definite relationships of physico-chemical properties such as temperature and salinity….
Stockman, W. B.. 1946. "A theory of T-S curves as a method for studying the mixing of water masses in the sea." Journal of Marine Research 6, (1). https://elischolar.library.yale.edu/journal_of_marine_research/626