1. The 18th letter of the Greek alphabet. See Table at alphabet.
2. A sigma factor.
3. Statistics The standard deviation of a given probability distribution or a given set of data.
[Greek sīgma, of Phoenician origin; see smk in the Appendix of Semitic roots.]
sigmate′ (-māt′) adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 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.