Not rotatory; passing from one point to another by a movement other than rotation; -- said of the movement of parts of a liquid or yielding mass.

An irrotational fluid flow is one whose streamlines never loop back on themselves. Typically, only inviscid fluids can be irrotational. Of course, a uniform viscid fluid flow without boundaries is also irrotational, but this is a special (and boring!) case.

Applied to a vector field having zero vorticity or curl throughout the field. Two equivalent properties of an irrotational field are that there is no circulation about any reducible curve within the fluid, and that a potential exists. An autobarotropic fluid is irrotational for all time if it is irrotational at any time. Meteorological motions of the smaller scales, for example, gravity waves, may be treated as irrotational, but when the scale is large enough to take the rotation of the earth into account, only rotational motions are of interest. See solenoidal, Helmholtz's theorem.