Coriolis Effect
Definition
The apparent curving of moving air and water caused by Earth's rotation — hurricanes spin opposite ways in the two hemispheres.
Technical definition
In a rotating frame a fictitious force F_cor = −2m(ω × v) acts perpendicular to the velocity. It is strongest at the poles and zero at the equator.
Example
An artillery shell fired north in the Northern Hemisphere lands east of its target — the ground rotated eastward beneath it during the flight.
Common confusion
The Coriolis effect does not decide which way your bathtub drains — at that scale it is far too weak. Drain swirl is set by the basin's shape and leftover motion.