Superfluidity, the frictionless flow and other exotic behaviour observed in liquid helium at temperatures near absolute zero (−273.15 °C, or −459.67 °F), and (less widely used) similar frictionless behaviour of electrons in a superconducting solid.

Superfluids are thought to flow endlessly, without losing energy, similar to electrons in a superconductor. Observing the behavior of superfluids therefore may help scientists improve the quality of superconducting magnets and sensors, and develop energy-efficient methods for transporting electricity.

A superfluid acts like a mixture of a normal fluid and a superfluid. As the temperature drops, more of the fluid is superfluid and less of it is an ordinary fluid. Some superfluids display high thermal conductivity.

In a superfluid, the particles that make up the fluid are in a state of matter known as a Bose-Einstein condensate (BEC). This means that they are all in the same quantum state and behave as a single entity.

Superfluids are a state of matter that behave like a fluid with zero viscosity or friction. There are two isotopes of helium that can create a superfluid.