an ancient Greek or Roman galley, usually a warship, with three banks of oars on each side
Webster’s New World College Dictionary, Fourth Edition, compiled by the
editors of Webster’s New World Dictionaries. Copyright © 2010 by Houghton
Mifflin Harcourt. Published by Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'trireme'
trireme (orig., having three banks of oars) < tri-, tri-
oar: see row3