1. Of, relating to, or occurring in the night: nocturnal stillness.
2. Botany Opening at night. Used of flowers.
3. Zoology Most active at night: nocturnal lizards.
[Middle English, from Old French, from Late Latin nocturnālis, from Latin nocturnus, from nox, noct-, night; see nekw-t- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 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.