Almost all letters of the Greek alphabet designate a mathematical concept. The first three letters -- alpha, beta and gamma -- are often used as substitutes for particular numbers. Theta is very commonly used for angles, while lambda, mu and nu designate constant multipliers. Pi is perhaps the most familiar Greek symbol in mathematics, designating the irrational number that is a constant value for any circle's ratio between circumference and diameter. Capital sigma is often used as the symbol of summation.
In physics, alpha stands for the Wein constant and it is also used with beta for designating particles. The coefficient of viscosity is represented by eta and the decay constant by lambda. Physicians use mu to designate both permeability and the coefficient of friction. Rho stands for density and reflexivity, while sigma is a symbol for tensile stress and conductivity.
In statistics, alpha is the symbol for a type I error, while beta stands for a type II error. The standard deviation for the population is represented by sigma. The arithmetic mean of the population is symbolized by the Greek letter mu.
In 1603, the German astronomer Johannes Bayer decided to use Greek symbols to designate the brightest stars in a constellation. Stars are given letters according to their brightness, beginning with alpha. For example, Alpha Orionis is the brightest star in the Orion constellation. Bayer's system included Roman letters after the 25 letters of the Greek alphabet, a practice that is nowadays hardly encountered.