Thalassa was the general word for 'sea' and for its divine female personification in Greek mythology. The word may have been of Pre-Greek origin.

The Pre-Greek substrate consists of the unknown pre-Indo-European language(s) spoken in prehistoric Greece before the advent of the Proto-Greek language in the Greek peninsula during the Bronze Age. It is possible that Greek acquired approximately 1,000 words from such a language or group of languages, because some of its vocabulary cannot be satisfactorily explained as deriving from Proto-Greek and a Proto-Indo-European reconstruction is almost certainly impossible for such terms.