from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The 18th letter of the Greek alphabet.
  • noun A sigma factor.
  • noun Statistics The standard deviation of a given probability distribution or a given set of data.

from The Century Dictionary.

  • noun The name of the Greek letter Σ, σ, ς, equivalent to the English S, s. (For its early forms, see under S.)
  • noun An S-shaped or sigmoid flesh-spicule of a sponge.
  • noun The curve of the letter S or any approximate curve.
  • noun In descriptive biol., one of the S-shaped ridges on the surface of certain molluscan shells, as those of gastropods and cephalopods.
  • noun In experimental psychology, the time-unit, of a second: abbreviated σ.

from the GNU version of the Collaborative International Dictionary of English.

  • noun The Greek letter Σ, σ, or ς (English S, or s). It originally had the form of the English C.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun The name for the eighteenth letter of the Classical and Modern Greek, the nineteenth letter of Old and Ancient.
  • noun mathematics The symbol Σ, used to indicate summation of a set or series.
  • noun statistics The symbol σ, used to indicate one standard deviation from the mean, particularly in a normal distribution.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun the 18th letter of the Greek alphabet


from The American Heritage® Dictionary of the English Language, 4th Edition

[Greek sīgma, of Phoenician origin; see smk in Semitic roots.]



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  • Greek C~shaped.

    July 11, 2008