A superfluid is a liquid that exhibits
zero viscosity; there is no interaction between the components of the fluid, i.e. the
atoms. An example of this is
liquid helium, cooled to 2
degrees above
absolute zero. Although the huge
pressures in the
core of a
neutron star may contain
matter in an exotic state which may also exhibit superfluidity, even though the temperature may be quite high.
In this state the entire volume of material can be modelled (in fact
has to be modeled) using
quantum mechanics.
A superfluid, once set in motion, will never stop flowing, this has some parallels to the phenomenon of
superconductivity.