1. The ordinal number matching the number three in a series.
2. One of three equal parts.
a. An interval of three degrees in a diatonic scale.
b. A tone separated by three degrees from a given tone, especially the third tone of a scale.
4. The transmission gear or gear ratio used to produce forward speeds next higher to those of second in a motor vehicle.
5. Baseball Third base.
6. thirds Merchandise whose quality is below the standard set for seconds.
[Middle English thridde, therdde, third, from Old English thridda; see trei- in the Appendix of Indo-European roots.]
third adv. & adj.
Word History: Every native speaker knows that the cardinal three and the ordinal third are closely related, but many may wonder why the r comes before the vowel in the former and after in the latter. What we have here is metathesis, the switching of the order of two sounds. This is a common occurrence in languages, and especially so in English with the consonant r. In Old English, three was thrīe, and third was thridda. Thridda would have given us thrid in Modern English except for the metathesis of r and i. This metathesis began in Old English times in Northumbria: thridda appears as thirdda in Northumbrian manuscripts. The metathesis spread south during Middle English times and also affected many other words, including bird (originally bridd in Old English and in Chaucer's Middle English), and nostril, literally "nose hole" (from Old English thyrl). Metathesis even produced the curious form throp from thorp, "village," which survives in the proper name Winthrop.
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.