[nep-too-nee-uh n, -tyoo-]
  1. pertaining to Neptune or the sea.
  2. pertaining to the planet Neptune.
  3. (often lowercase) Geology. formed by the action of water.

Origin of Neptunian

First recorded in 1650–60; Neptune + -ian
Related formstrans-Nep·tu·ni·an, adjective Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for neptunian

Historical Examples of neptunian

British Dictionary definitions for neptunian


  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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for neptunian



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.

Online Etymology Dictionary, © 2010 Douglas Harper