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(fluid with no viscosity)

A fluid (gas or liquid) is a superfluid if it has absolutely no viscosity (no friction between adjacent portions of the fluid), the characteristic being termed superfluidity. One consequence is that movements such as vortices don't slow and drag, but continue forever. Helium in its liquid state is an example (or close to a superfluid: like many concepts, superfluidity is an ideal). Astrophysical models of some phenomena have included superfluidity, i.e., some theories of neutron star internals.

An oft-demonstrated phenomenon of liquid helium's superfluidity is its tendency to flow out of a glass beaker, up the sides, over the rim, and down. This occurs because despite liquid helium's lack of viscosity, it is attracted to the glass (as are some other liquids, such as water), but liquid helium's lack of viscosity removes one impediment to lifting higher above the liquid surface, allowing it to creep upward, reach, and pass over the rim of the container. This eventually acts like a siphon, constrained not by the sides of a pipe but by the glass surface and the force drawing the liquid toward that surface. Liquid helium happens to adhere to glass, but does not adhere to all materials, and containers of these other materials do not show this effect. The apparent gravity-defying property is a combination of superfluidity and the adhesion properties of glass with liquid helium.

Some models of dark matter suggest it is or is sometimes a superfluid.

(fluid dynamics)
Further reading:

Referenced by pages:
Bose-Einstein condensate (BEC)
Bose-Einstein statistics
neutron degenerate matter
neutron star (NS)
supercritical fluid (SCF)