1. Roman Mythology The god of the dead and the ruler of the underworld, identified with the Greek Hades.
2. A dwarf planet having a sidereal period of revolution about the sun of 248.5 years, a highly elliptical orbit with a perihelion distance of 4.4 billion kilometers (2.8 billion miles) and an aphelion distance of 7.4 billion kilometers (4.6 billion miles), and a mean equatorial diameter of 2,302 kilometers (1,485 miles), less than half that of Earth. Until 2006, Pluto was classified as the ninth planet in the solar system. See Usage Note at planet.
[Latin Plūtō, Plūtōn-, from Greek Ploutōn, from ploutos, wealth (from the belief that the underworld was the source of wealth from the ground); see pleu- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.