Sigma
WordNet
noun
(1) The 18th letter of the Greek alphabet
WiktionaryText
Noun
- The name for the eighteenth letter of the Classical and Modern Greek, the nineteenth letter of Old and Ancient.
- the symbol (Σ) is used to indicate summation of a set or series.
- the symbol (σ) is used to indicate one standard deviation from the mean, particularly in a normal distribution.