Bose-Einstein condensate (BEC) is a state of matter (analogous to solid, liquid, gas, or plasma) that can occur when bosons are at an extremely low energy level (extremely low temperature). The state was theorized by Albert Einstein based upon Bose-Einstein statistics and quantum mechanics, and was first produced in a lab in 1995 using recently-developed techniques of cooling extremely thin gas to very close to absolute zero. Among its characteristics are quantum characteristics on much larger scales than typically under consideration.
Bose-Einstein condensate is not the same thing as a superfluid, the latter which is defined to be a fluid having no viscosity. Helium cooled to its liquid state is a superfluid but the average quantum state of the particles is generally too high to consider it a BEC. The BECs that have been created and many that have been theorized are superfluids.
Some models of observed astronomical phenomena incorporate types of Bose-Einstein condensate, including models of dark matter.