TRITON

I. noun Etymology: Latin, from Greek Tritōn Date: 1536 a son of Poseidon described as a demigod of the sea with the lower part of his body like that of a fish, any of various large marine gastropod mollusks (family Ranellidae syn. Cymatiidae) with a heavy elongated conical shell, II. noun Etymology: tritium + 2-on Date: 1934 the nucleus of tritium