Superfluidity is the sudden lowering of viscosity together with an increase in the thermal conductivity of a substance. Below a certain temperature viscosity, which is related to molecular collisions, decreases dramatically, as Einstein was the first to point out. Superfluids flow without friction.
Ordinary helium (4), a quantum liquid composed of bosons, appears below 2 kelvins. This quantum phenomenon which appears on a macroscopic scale has been known since 1938 (Kapitsa, Nobel prize 1978). For their discovery of the superfluidity of helium 3 (a fermion) David Mac Lee, Robert C. Richardson and Douglas D. Osherhoff were awarded the Nobel prize (1972).
Superfluidity has various similarities with superconductivity (formation of Cooper electron pairs).