1. Occurring or completed without perceptible delay: Relief was instantaneous.
2. Done or made as quickly or directly as possible: an instantaneous reply to my letter.
3. Present or occurring at a specific instant: instantaneous velocity; instantaneous pressure.
[From Medieval Latin īnstantāneus, from Latin īnstāns, īnstant-, present; see INSTANT.]
in·stan′ta·nei·ty (ĭn-stăn′tə-nēĭ-tē, ĭn′stən-) n.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 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.