Definition of "sigma"

sigma

/ˈsɪɡmə/

noun
plural sigmas or sigmata
1. The eighteenth letter of the Classical and Modern Greek alphabets, the twentieth letter of Old and Ancient.
  Quotations
  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.
  Quotations
  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.
  2011, Paul Rincon, (bbc.co.uk) LHC: Higgs boson 'may have been glimpsed'
4. Clipping of sigma male.