in physics, a functional relationship for a system of particles that is useful for calculating the overall statistical and thermodynamic behaviour of the system without explicit reference to the detailed behaviour of particles. The canonical ensemble was introduced by J. Willard Gibbs (Gibbs, J Willard), a U.S. physicist, to avoid the problems arising from incompleteness of the available observational data concerning the detailed behaviour of a system of interacting particles—for example, molecules in a gas.

One way to describe a system of particles is to state explicitly the position and momentum (i.e., mass times velocity) of each particle. If there are N particles and each particle has s modes in which it can move (see freedom, degree of ), 2sN values are required to specify its state. This system can then be described as a point in a 2sN-dimensional space (called gamma 【Γ】 space). As time passes, changes in the details of the system would correspond to movement of the point in the Γ space. An ensemble is a large number of similar systems, as described by a collection of points in Γ space.

A canonical ensemble (or, more explicitly, macrocanonical ensemble) is an ensemble for which the density of points in Γ space varies exponentially with the total energy E of the system: ρ = Ae^-E/θ, in which A and theta (θ) are constants of the system. If the system is in equilibrium at absolute temperature T, its gross (macroscopic) behaviour will be described by taking the average behaviour of a system in a canonical ensemble in which θ = kT. The constant k is called Boltzmann's constant.

A microcanonical ensemble consists of systems all of which have the same energy and is often found useful in describing isolated systems in which the total energy is a constant. Such macrocanonical and microcanonical ensembles are examples of petit ensembles, in that the total number of particles in the system is specified.

A grand ensemble is any ensemble for which the restriction of a constant number of particles is abandoned. Such a description is more general and is particularly applicable to systems in which the number of particles varies, e.g., chemically reacting systems.