The Collaborative International Dictionary
Vortex ring \Vortex ring\ (Physics) A ring-shaped mass of moving fluid which, by virtue of its motion of rotation around an axis disposed in circular form, attains a more or less distinct separation from the surrounding medium and has many of the properties of a solid.
Wikipedia
A vortex ring, also called a toroidal vortex, is a torus-shaped vortex in a fluid; that is, a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, more precisely poloidal.
Vortex rings are plentiful in turbulent flows of liquids and gases, but are rarely noticed unless the motion of the fluid is revealed by suspended particles—as in the smoke rings which are often produced intentionally or accidentally by smokers. Fiery vortex rings are also a commonly produced trick by fire eaters. Visible vortex rings can also be formed by the firing of certain artillery, in mushroom clouds, and in microbursts.
A vortex ring usually tends to move in a direction that is perpendicular to the plane of the ring and such that the inner edge of the ring moves faster forward than the outer edge. Within a stationary body of fluid, a vortex ring can travel for relatively long distance, carrying the spinning fluid with it.
Usage examples of "vortex ring".
The vortex ring struck harmlessly against the screen, was absorbed by it.
A planecutting toroidal vortex ring turns flat objects inside out.
But all that happened was that a shining silver vortex ring emerged from the gun, shot swiftly toward the capsule, and bounced off it harmlessly, still expanding.