In radio transmission, multipath refers to the simultaneous reception of two copies of the signal, that arrive via separate paths with different delays.
A common example is when a signal bounces off a building or other object and is received along with the direct (unbounced) signal. In television reception, this causes "ghosting" -- one sees a faded echo on the screen horizontally displaced from the main image.
Another common example is in radio (especially AM radio), where the signal bounces off the ionosphere and one receives that delayed signal along with the directly transmitted signal.
Usually, multipath is an undesired effect but in MIMO systems, separate antennas deliberately send replicas and sophisticated receivers piece together the fragments to improve performance.
