sigma

Etymology

From Ancient Greek σῖγμα (sîgma), σίγμα (sígma), probably from the root sig- as seen in σίζω (sízō) (< sig-yo "to hiss") + -μα (-ma).

noun

1. The eighteenth letter of the Classical and Modern Greek alphabets, the twentieth letter of Old and Ancient.
   Their names, which are expressive of some bodily or mental excellence, all end with the same letter---the letter which is called San by the Dorians, and Sigma by the Ionians. "Herodotus: On The Customs of the Persians", I.139
2. (mathematics) The symbol Σ, used to indicate summation of a set or series.
3. (statistics) The symbol σ, used to indicate one standard deviation from the mean, particularly in a normal distribution.
   2011, Paul Rincon, (bbc.co.uk) LHC: Higgs boson 'may have been glimpsed' None of the spikes seen by the experiments is at much more than the "two sigma" level of certainty. A level of "five sigma" is required to claim a discovery, meaning there is less than a one in a million chance the data spike is down to a statistical fluke.
4. Clipping of sigma male.