A superfluid is described as a macroscopically coherent quantum system S_F that exhibits ‘unusual’ properties such as negligible losses or no resistance to flows. Therefore, it is defined as a phase that exhibits most unusual effects, previously considered to be anomalous from a Thermodynamic viewpoint, that can be however explained in terms of quantum statistical theory/quantum statistical mechanics and long-range coherence of coupled boson systems. Such theories are, for example, L. D. Landau’s phenomenological/ microscopic theory of superfluidity in liquid ⁴He (the 1962 Nobel Prize in Physics), and Sir Anthony Leggett’s quantum statistical theory of superfluidity in liquid ³He (the 2003 Nobel Prize in Physics shared with Alexei A. Abrikosov).

As an example, in a superconductor the long-ranged coupled Cooper electron pairs form a superfluid that sustains very high electric currents without any significant heating of the metal. Liquid ³He is another example of a superfluid where the liquid flow can take place upwards in the absence of externally applied pressure and no apparent resistance to flow–no viscosity.