neptunian

adjective

1. pertaining to Neptune or the sea.
2. pertaining to the planet Neptune.
3. (often lowercase) Geology. formed by the action of water.

adjective

1. of or relating to the Roman god Neptune or the sea
2. of, occurring on, or relating to the planet Neptune
3. geology (of sedimentary rock formations such as dykes) formed under water

adj.1650s as “pertaining to the god Neptune;” 1794 in the geological sense, referring to actions of water, from Neptune + -ian. Usually opposed to volcanic or plutonic. As a noun meaning “inhabitant of the planet Neptune” it is recorded from 1870.