adj. rap·id·er, rap·id·est
Moving, acting, or occurring with great speed. See Synonyms at fast1.
n. often rapids
An extremely fast-moving part of a river, caused by a steep descent in the riverbed.
[Latin rapidus, from rapere, to seize; see rep- in the Appendix of Indo-European roots.]
ra·pidi·ty (rə-pĭdĭ-tē), rapid·ness (răpĭd-nĕs) n.
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.