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.
New to topics? Read the docs here!