Sigma

Sigma is the eighteenth letter of the Greek alphabet. Uppercase sigma (Σ) is used in Sigma Notation, also known as summation notation in mathematics. It represents the addition of a sequence, and this sequence can be derived from values given by a function, values in a set, values present in a Free-Body Diagram, etc. Lowercase sigma (σ) represents many things across all sciences. Notably, it represents the Stefan-Boltzmann constant, a constant used in the Stefan-Boltzmann law of radiation. In chemistry, sigma bonds are a type of bond characterized by overlapping p-orbitals. In quantum mechanics, sigma represents Pauli matrices. In biology, sigma receptors are poorly understood cell receptors that bond to certain stimulants like cocaine, meth, and other drugs.