function in number theory the empty string in formal grammar a formal system in mathematical logic thermal conductivity the Lorentz transformation Μμ (mu) represents: the Möbius function in number theory the population mean or expected value in probability and statistics a measure in measure theory micro-, an SI prefix denoting 10−6 (one millionth) the coefficient of friction in physics the service rate in queueing theory the dynamic viscosity in physics magnetic permeability in electromagnetics a muon reduced mass the ion mobility in plasma physics the Standard gravitational parameter in celestial mechanics a micron or micrometer (10−6 meter) (common usage in biology, though officially revoked in 1967) population mean in statistics chemical potential in thermodynamics Absolute refractive index of a medium in optics Νν (nu) represents: frequency in physics in hertz (Hz) Poisson's ratio in material science a neutrino kinematic viscosity of liquids stoichiometric coefficient in chemistry true anomaly in celestial mechanics degrees of freedom in statistics the matching number of a graph the p-adic valuation of a number Ξξ (xi) Ξ represents: the original Riemann Xi function, i.e. Riemann's lower case ξ, as denoted by Edmund Landau and currently the xi baryon represents: the original Riemann Xi function the modified definition of Riemann xi function, as denoted by Edmund Landau and currently correlation length in physics Οο (omicron) Ππ (pi) Π represents: the product operator in mathematics a plane the unary projection operation in relational algebra osmotic pressure represents: Archimedes' constant (more commonly just called Pi), the ratio of a circle's circumference to its diameter the prime-counting function the state distribution of a Markov chain in reinforcement learning, a policy function defining how a software agent behaves for each possible state of its environment a type of covalent bond in chemistry (pi bond) a pion (pi meson) in particle physics in statistics, the population proportion nucleotide diversity in molecular genetics in electronics, a special type of small signal model is referred to as a hybrid-pi model in relational algebra for databases, represents projection ϖ (a graphic variant, see pomega) represents: angular frequency of a wave, in fluid dynamics (angular frequency is usually represented by but this may be confused with vorticity in a fluid dynamics context) longitude of pericenter, in astronomy comoving distance, in cosmology Ρρ (rho) Ρ represents: one of the Gegenbauer functions in analytic number theory (may be replaced by the capital form of the Latin letter P).