An ensemble is a concept of statistical mechanics to designate a collection of microtoics in which a system can be located. From the ergodic principle, which states that each microtoxic state is equally likely, it can be deduced in which macros mode the system is most likely to be.
The statistical mechanics make frequent use of ensembles to express the uncertainty about the microscopic details of the state of a system with a large number of particles. For example, in a laboratory one can keep the temperature of a gas within rather narrow limits, but one can not control the position and velocity of each individual molecule. Thus, the state of the gas is represented by a distribution function over a large number of possible combinations of positions and velocities, which however all yield the same macroscopic temperature.
Commonly used ensembles are:
The first two examples are statistical distributions on the phase space, which is the combination of all possible (generalized) positions and pulses of a given set of particles.
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