A superfluid is described as a macroscopically coherent quantum system 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 He (the 1962 Nobel Prize in Physics), and Sir Anthony Leggett's quantum statistical theory of superfluidity in liquid He (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 He 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.