A type of flow pattern exhibited by superfluids, such as liquid ⁴He below 2.17 K. The term vortex designates the familiar whirlpool pattern where the fluid moves circularly around a central line and the velocity diminishes inversely proportionally to the distance from the center. The strength of a vortex is determined by the circulation k, which is the line integral of the velocity around any path enclosing the central line.

For an ordinary vortex, k can possess any value; for a superfluid vortex, k is restricted to a quantized multiple of Planck's constant h divided by m, the mass of the helium atom. Hence the expression quantized vortex line. Although h and m are microscopic (that is, atomic) quantities, their ratio h/m is rather large, being equal to 10⁻³ cm² · s⁻¹.