1. The third letter of the Greek alphabet. See Table at alphabet.
2. The third item in a series or system of classification.
3. A unit of magnetic intensity equal to one hundred thousandth (10-5) of an oersted.
4. A unit of mass equal to one millionth (10-6) of a gram.
1. Being in the third position relative to a designated carbon atom in an organic molecule at which an atom or a group may be substituted.
2. Referring to the third of a group of isomers, or molecules of similar origin or properties, determined arbitrarily by those who discover or classify them. Used in combination.
[Middle English, from Greek, of Phoenician origin; see gml2 in the Appendix of Semitic roots.]
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.