The victim is forced against any spiky object that passes sufficiently deeply into the body so as to *really* hurt, and preferably pin them to another object.

Note that if the spiky object is a weapon then it won't qualify as impalement if it is simply used in the normal manner. For example, getting stabbed by a spear isn't necessarily impalement, but being kicked onto the end of a spear that is stuck through a door (Enter the Dragon (qv)) would qualify.

Another common example that doesn't qualify is being struck with arrows.