Probability theory is a branch of mathematics that deals with calculating the likelihood of a given event’s occurrence, which is expressed as a number between 1 and 0. (Source: http://whatis.techtarget.com/definition/0,,sid9_gci549076,00.html)
See also:Formalism, Stochastic Music
Bibliography: ASUAR, José Vicente (1972a). Música con Computadores: ¿cómo hacerlo..?