rille also rill (ril)
1. A long, narrow, roughly straight channel or groove on the surface of a solar system body like a planet or moon.
2. Variant ofrill.
[GermanRille; see rei- in the Appendix of Indo-European roots, fromLow Germanrille; see rei- in the Appendix of Indo-European roots.]
(click for a larger image)rille
Ariadaeus, a lunar rille, photographed in 1969 by the crew of Apollo 10
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.