n. pl. mil·len·ni·a (-lĕnē-ə) or mil·len·ni·ums
1. A span of one thousand years.
2. A thousand-year period of holiness mentioned in the book of Revelation, during which Jesus and his faithful followers are to rule on earth.
3. A hoped-for period of joy, serenity, prosperity, and justice.
4. A thousandth anniversary.
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.