= Statistical ensemble
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A <statistical ensemble> describes hypothetical copies of the same system under specified constraints, with a <probability distribution> over their <microstates>. An <ensemble average> is an <expected value> across those copies. The <canonical ensemble> fixes <temperature> and particle number while allowing <energy> exchange; the <microcanonical ensemble> fixes <energy>, and the <grand canonical ensemble> also allows particle exchange. These choices specify different physical preparations. https://en.wikisource.org/wiki/Elementary_Principles_in_Statistical_Mechanics[Gibbs (1902), preface: ensembles of systems with different microscopic conditions]{ref}
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