A state of matter in which atoms or particles are chilled to such low energies that they ‘condense’ into a single quantum state.The atoms are bosonic in nature; that is, their total spin must possess an integral value, such as 0, 1, 2. Particles, like everything, have wave properties, such as wavelength. The trick is getting into a regime where the wave properties emerge. The wavelength (called Debroglie wavelength) of an atom is related to its temperature—the colder the atom, the longer the wavelength. At room temperature, atoms can be treated like billiard balls bouncing around. At low temperatures, the wavelengths become longer, and so the wave properties become relevant. For sufficiently low temperature, a few millionths of a degree above zero temperature, the bosonic atoms effectively become overlapping waves that share the same phase. The atoms become a BEC, which has quantum mechanical behavior.
The BEC phenomenon was first predicted by Satyendra Bose and Albert Einstein in the 1920s, hence the name. BEC was first noted to exist in liquid helium. For their observation of BECs in dilute neutral atomic gases, Eric Cornell of NIST/JILA, Wolfgang Ketterle of MIT, and Carl Wieman of Colorado/JILA received the 2001 Nobel Prize in physics. While BEC is most often used in the context of neutral atoms, the boson particle can of course be something other than an atom. Examples of these non-atomic species include Cooper pairs, pairs of electrons participating in the superconducting state; or excitons, consisting of an electron-hole pair.