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Lower-case σ is used for:

the standard deviation of a population or probability distribution in statistics
sigma bonds in chemistry
to represent an unknown angle in mathematics
cross section (physics)
velocity dispersion in astronomy
sigma constant in organic chemistry
the sigma receptor in biology
a quality model for business, Six Sigma (6σ), based on the standard deviation
sigma-algebras, sigma-fields, and sigma-finiteness in measure theory; in general terms, the symbol σ serves as a shorthand for "countably", e.g. a σ-compact topological space is one that can be written as a countable union of compact subsets.
the generated sigma-algebra of a set A {\displaystyle A} A is denoted σ ( A ) {\displaystyle \sigma (A)} \sigma (A)
the sum-of-divisors function in number theory
the Stefan–Boltzmann constant
the "sigma factor" of RNA polymerase
a measure of electrical conductivity
the surface charge density in electrostatics
normal stress in continuum mechanics
volatility of a stock generally needed for options pricing
a syllable in phonology
the spectrum of a matrix A {\displaystyle A} A, denoted as σ ( A ) {\displaystyle \sigma (A)} \sigma (A), in applied mathematics
surface tension
the unary operation of selection on a database relation in relational algebra
the Pauli matrices in quantum mechanics
a target's radar cross-section (RCS) in radar jamming or electronic warfare
the life span of a basic multicellular unit (BMU) in bone remodeling
the damping parameter in signal processing
a millisecond in early 20th-century physiology literature[5]
the Weierstrass sigma-function
the Sigmoid/Logistic function in machine learning