= Ultracold gas
An <ultracold gas> is a dilute gas cooled until atomic motion becomes slow enough that <quantum mechanics> is essential to its preparation, collisions or collective state. There is no single <temperature> boundary independent of particle mass and <number density>: the overlap of thermal matter waves depends on the <thermal de Broglie wavelength> relative to interparticle spacing.
<Laser cooling> can prepare a cold trapped sample, and <evaporative cooling of atoms> can further increase its <phase-space density of an atomic gas>. Expansion after releasing a trap gives information about the <velocity> distribution. A small measured cloud alone is insufficient evidence for low <temperature>, because spatial confinement also depends on the trapping <potential energy>.
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