nep·tune \ˈnepˌt(y)ün sometimes -pˌchün\noun (-s) Etymology: after Neptune, Roman god of waters, from Latin Neptunus 1.usually capitalized: ocean, sea < full often hath she … sat with me on Neptune's yellow sands — Shakespeare > 2.: a copper or brass plate or pan used in trade with the natives of Africa 3.orneptune greenoften capitalized N: a light to moderate green that is bluer and stronger than surf green