Definition of sigma in US English:



  • 1The eighteenth letter of the Greek alphabet (Σ, σ), transliterated as ‘s.’.

    The form ς is used instead of σ at the end of a word. The uncial form, called lunate sigma and resembling the letter C, is also sometimes used

    • ‘You know you have seen them - those people who walk around wearing paraphernalia emblazoned with odd symbols like phi, sigma, and pi.’
    • ‘Alpha, rho, tau, sigma, omega, Ash went over the letters he saw in his mind while Lari just stared, apparently in her own little world.’
    • ‘He coined this latter term and employed the Greek letter lower case sigma to denote its population parameter.’
    • ‘Finally, even if epsilon is the correct reading, in the letter space to its right, where Meritt read part of an iota, are traces of what appears to be a sigma.’
  • 2SigmaAstronomy
    followed by Latin genitive The eighteenth star in a constellation.

    ‘Sigma Octantis’
    • ‘Finally, the final green beam of light was shot straight up at the constellation Sigma.’
  • 3Chemistry Physics
    Relating to or denoting an electron or orbital with zero angular momentum around an internuclear axis.

    • ‘The sigma bond occupies the internuclear space, and the two pi bonds are at 90° angles to each other surrounding the carbon-carbon internuclear space.’
    • ‘The sigma particles can be positive, negative, or neutral.’
    • ‘These conical intersections are fundamentally different from the ‘H-vinyl’ structures found in simple alkenes because they involve the interaction of four sigma electrons, as opposed to two pi electrons and two sigma electrons.’
    • ‘The triple bond is composed of one sigma bond from overlap of sp hybrid orbitals, and two pi bonds, from overlap of two sets of p orbitals.’

  • 1Mathematical sum.

  • 2Standard deviation.