In
mathematical physics, especially as introduced into
statistical mechanics and
thermodynamics by
J. Willard Gibbs in 1902, an
ensemble (also
statistical ensemble) is an idealization consisting of a large number of virtual copies (sometimes infinitely many) of a
system, considered all at once, each of which represents a possible state that the real system might be in. In other words, a statistical ensemble is a probability distribution for the state of the system.