Of or relating to the deepest regions of the ocean, below about 6,000 meters (20,000 feet).
[French, from Hadès, Hades, from Greek Haidēs; see weid- 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.