(n. fallthrough, var.: fall-through)
1. To exit a loop by exhaustion, i.e., by having fulfilled its exit condition rather than via a break or exception condition that exits from the middle of it. This usage appears to be really old, dating from the 1940s and 1950s.
2. To fail a test that would have passed control to a subroutine or some other distant portion of code.
3. In C, ‘fall-through’ occurs when the flow of execution in a switch statement reaches a case label other than by jumping there from the switch header, passing a point where one would normally expect to find a break. A trivial example:
/* FALL THROUGH */
The variant spelling /* FALL THRU */ is also common.
The effect of the above code is to do_green() when color is GREEN, do_red() when color is RED, do_blue() on any other color other than PINK, and (and this is the important part) do_pink() and then do_red() when color is PINK. Fall-through is considered harmful by some, though there are contexts (such as the coding of state machines) in which it is natural; it is generally considered good practice to include a comment highlighting the fall-through where one would normally expect a break. See also Duff's device.