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Ensemble (mathematical physics)
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<p>In <a href="/facts/Physics/a7aH2y08">physics</a>, specifically <a href="/facts/Statistical_mechanics/6zMsNYYG">statistical mechanics</a>, an ensemble (also statistical ensemble) is an idealization consisting of a large number of virtual copies (sometimes infinitely many) of a <a href="/facts/System/hmqQ4ASX">system</a>, 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 set of systems of particles used in statistical mechanics to describe a single system. The concept of an ensemble was introduced by <a href="/facts/Willard_Gibbs/xyWwxQtA">J. Willard Gibbs</a> in 1902. </p><p>A thermodynamic ensemble is a specific variety of statistical ensemble that, among other properties, is in statistical equilibrium (defined below), and is used to derive the properties of <a href="/facts/Thermodynamic_system/VUr0kpWu">thermodynamic systems</a> from the laws of classical or quantum mechanics. </p>