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 MusicBibliography: XENAKIS, Iannis (1958). Auf der Suche nach einer Stochastischen Musik
XENAKIS, Iannis (1960-1961). Grundlagen einer stochastischen Musik
XENAKIS, Iannis (1966). Zu einer Philosophie der Musik

