A quantum phenomenon in a system of bosons consisting in the fact that, at a temperature below the so-called degeneracy temperature, part of the particles of the system accumulates in the state with zero momentum (if the system as a whole is at rest); it is named by analogy with the process of condensation of vapour molecules into a liquid as the vapour cools. No condensation in the ordinary sense occurs here, however: the distribution of the particles in space remains as it was, and what is involved is only condensation in momentum space. For the overwhelming majority of gases the degeneracy temperature is very low, and the substance passes into the solid state long before Bose–Einstein condensation can set in. The exception is helium, which under normal conditions passes into the liquid state at T=4.2 K and remains a liquid down to the temperatures closest to absolute zero. At T=2.17 K and the saturated vapour pressure liquid 4He passes into the superfluid state, whose appearance can be connected with Bose–Einstein condensation. (see Quantum liquid, Superfluidity).