Statistical mechanics is a branch of physics that deals with the statistical behavior of particles or systems, often in thermodynamic equilibrium. There are several synonyms for the term statistical mechanics, including statistical physics, equilibrated statistical mechanics, and ensemble theory. Statistical physics refers to the study of thermal and kinetic properties of matter in terms of the behavior of large numbers of individual atoms, molecules, or particles. Equilibrated statistical mechanics refers to the study of systems that have reached equilibrium, where the properties of the system no longer change with time. Ensemble theory refers to the use of statistical methods to describe the properties of a large number of systems.