It was first defined and used in physics (in particular in statistical mechanics) for describing particle speeds in idealized gases. In an idealized gas the particles move freely inside a stationary container without interacting with one another, most of the time. Sometimes they collide and exchange energy and momentum with each other or with their thermal environment. Particle in this context refers to gaseous particles (atoms or molecules), and the system of particles is assumed to have reached thermodynamic equilibrium.