Sigma (letter) in the context of "Summation"

Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). For example, if a 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

Electrical conductivity (or specific conductance) is the reciprocal of electrical resistivity. It represents a material's ability to conduct electric current. It is commonly signified by the Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) and γ (gamma) are sometimes used. The SI unit of electrical conductivity is siemens per metre (S/m). Resistivity and conductivity are intensive properties of materials, giving the opposition of a standard cube of material to current. Electrical resistance and conductance are corresponding extensive properties that give the opposition of a specific object to electric current.

Shin (also spelled Šin (šīn) or Sheen) is the twenty-first and penultimate letter of the Semitic abjads, including Phoenician šīn 𐤔, Hebrew šīn ש‎, Aramaic šīn 𐡔, Syriac šīn ܫ, and Arabic sīn س‎.

The Phoenician letter gave rise to the Greek Sigma (Σ) (which in turn gave rise to the Latin S, the German ẞ and the Cyrillic С), and the letter Sha in the Glagolitic and Cyrillic scripts (, Ш). The South Arabian and Ethiopian letter Śawt is also cognate. The letter šīn is the only letter of the Arabic alphabet with three dots with a letter corresponding to a letter in the Northwest Semitic abjad or the Phoenician alphabet.

Stigma (Ϛ ϛ) is a ligature of the Greek letters sigma (Σ) and tau (Τ), which was used in writing Greek between the Middle Ages and the 19th century. It is also used as a numeral symbol for the number 6. In this unrelated function, it is a continuation of the old letter digamma (originally Ϝ, cursive form ), which had served as a numeral since antiquity and was conflated with the σ-τ ligature in the minuscule handwriting of the Middle Ages.