sea serpent

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A large snakelike marine animal often reported by mariners since antiquity but never positively identified.

Century Dictionary and Cyclopedia

1. n. An enormous marine animal of serpentine form, said to have been repeatedly seen at sea. Most stories of the sea-serpent are obviously mythical. The few accounts which appear to have some foundation in fact have exhausted all possible conjectures respecting any actual creature. Some natnralists have suspected that a huge marine reptile may have survived from a former fauna; but certainly no animal is known which answers to any current conception of the sea-serpent, nor has such an animal ever been captured. The popular statements regarding sea-serpents are generally believed to be based on inaccurate observations of various large marine animals or of schools of animals.
2. n. In herpetology, a general name of the marine venomous serpents or seasnakes of the family Hydrophidæ. There are several genera and species, of warm seas, and especially of the Indian ocean, all extremely poisonous. The best-known belong to the genera Platurus, Pelamis, and Hydrophis, and have the tail more or less compressed like a fin. See also cuts under Hydrophis and Platurus.
3. n. A chain of salps linked together.
4. n. A name of Ophichthys gomesii, an eel of the family Ophichthyidæ, found in the West Indies.

Wiktionary

1. n. Any unidentified sea monster, especially one resembling a snake or dragon.

GNU Webster's 1913

1. (Zoöl.) Any marine snake. See sea snake.
2. (Zoöl.) A large marine animal of unknown nature, often reported to have been seen at sea, but never yet captured.

WordNet 3.0

1. n. huge creature of the sea resembling a snake or dragon