Superfluid is the state of matter that exhibits frictionless flow, zero viscosity, also known as inviscid flow.
To date, helium is the only fluid to exhibit superfluidity that has been discovered. Helium-4 becomes a superfluid once it is cooled to below 2.2K, a point known as the lambda point. At temperatures above the lambda point, helium exists as a liquid exhibiting normal fluid dynamic behavior. Once it is cooled to below 2.2K it begins to exhibit quantum behavior. For example, at the lambda point there is a sharp increase in heat capacity, as it is continued to be cooled, the heat capacity begins to decrease with temperature.  In addition, the thermal conductivity is very large, contributing to the excellent coolant properties of superfluid helium. Similarly, Helium-3 is found become a superfluid at 2.491mK.