1. The cardinal number equal to 7 × 10.
a. A decade or the numbers from 70 to 79: They became grandparents in their seventies. The temperature hovered in the seventies.
b. often Seventies The decade from 70 to 79 in a century.
[Middle English, from Old English -seofontig (in hundseofontig, the decade seventy, seventy); see sept in the Appendix of Indo-European roots.]
seven·ty 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.