n. pl. thir·ties
1. The cardinal number equal to 3 × 10.
a. A decade or the numbers from 30 to 39: They settled down in their thirties. The temperature fell into the thirties.
b. often Thirties The decade from 30 to 39 in a century.
3. An indication of the end of a news story, usually written 30.
4. Sports The second point that is scored by one side in tennis.
[Middle English thritty, thirty, from Old English thrītig; see trei- in the Appendix of Indo-European roots.]
thirty adj. & pron.
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.