Not a true force, but an side-effect of the rotation of a body. Objects moving in a rotating environment experience a force at right angles to its velocity and the rotation axis. In the case of the rotation of a planet around its axis, storms rotate counter-clockwise in the Northern and clockwise in the Southern Hemispheres. In a small orbital or rotating ship (such as a 100 meter diameter cylinder simulating 1 G), the Coriolis force causes a sideways movement in an object that falls to the floor (the outer wall) which can be quite disorientating. For this reason, rotating habitats tend to be large enough that only a slow rotation is required.
Hence: Coriolis drift departure from a straight-line trajectory, perceived by an observer in a rotating system; Coriolis effects in clouds were early evidence of the Earth's rotation.
Named for the Old Earth French engineer/mathematician Gustave-Gaspard Coriolis who discovered the effect in 1835.