Dictionary.net

[L.] (Zool.) Hydrophidae, or venomous sea-snakes, ranging to ten feet in length; abound in the Indian and Chinese seas. The great sea-serpent, ranging, it is said, to 600 feet in length, has hitherto, whenever thoroughly investigated, proved a delusion.

n. A serpent-like animal of great size, supposed to dwell in the sea—now reckoned as fabulous;—a kind of eel found in the Mediterranean; — a large marine serpent found in the Australian seas.