In physics, a statistical ensemble is a very large set of similar systems, considered all at once.

The topic of statistical ensembles is important in thermodynamics, statistical mechanics and quantum physics.

In quantum physics, there are two types of ensembles:

• Pure ensembles, which cannot be decomposed into different sub-ensembles. A ray in a Hilbert space is representing them in quantum mechanics.
• Mixed ensembles - decomposable into few different ensembles. A density matrix represents them in quantum mechanics. The decomposition into subensembles are many, and cannot be refined to a common one in general.

See also:

• density matrix
• Partition function (statistical mechanics)
• grand canonical ensemble