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.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.