A gas of atoms, generally ultracold, in which the individual atoms possess half-integer spin (that is, are fermionic). Since the 1980s, researchers have developed powerful new techniques for cooling atoms to ultralow temperatures. Among the most significant achievements made possible by these technical developments was the creation in 1995 of the long-sought Bose-Einstein condensate. In a Bose-Einstein condensate, atoms are cooled to such a low temperature that they collect in the quantum-mechanical ground state of their confinement volume. Bose-Einstein condensation is possible only in the class of particles known as bosons. Particles of the other class, fermions, are forbidden to occupy the same quantum state and are thus prevented from directly condensing.