Superfluid2 2009-05-09 09:38:02 2009-05-09 09:41:53 Definition supefluid $^3$He Cooper (electron) pairs long-range coupling superconductivity A {\em superfluid} is described as a highly coherent quantum system $S_F$ that exhibits `unusual' properties such as negligible losses or no resistance to flows. As an example, in a superconductor the long-ranged coupling of Cooper electron pairs form a superfluid that sustains very high electric currents without any significant heating of the metal. Liquid $^3$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.