sea-nymph, in Greek mythology, late 14c., Nereides (plural), via Latin from Greek Nēreis (genitive Nēreidos), daughter of the ancient sea-god Nēreus, son of Pontus and Gaia, husband of Doris, whose name is related to naros "flowing, liquid, I flow" (see Naiad). In zoology, "a sea-centipede" (1840).
The most famous among them were Amphitrite, Thetis, and Galatea. The Nereids were beautiful maidens helpful to voyagers, and constituted the main body of the female, as the Tritons did of the male, followers of Poseidon or Neptune. They were imagined as dancing, singing, playing musical instruments, wooed by the Tritons, and passing in long processions over the sea seated on hippocamps and other sea-monsters. Monuments of ancient art represent them lightly draped or nude, in poses characterized by undulating lines harmonizing with those the ocean, and often riding on sea-monsters of fantastic forms. [Century Dictionary]