1. The third letter of the Greek 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.
5. Chemistry. The third position from a designated carbon atom in an organic molecule at which an atom or a radical may be substituted.

[Middle English, from Greek, of Phoenician origin.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.