A character being hit over the head in a comedic manner and/or accompanied by the eponymous sound effect.
The purpose is often to dissuade the character from some course of action, or to punish them after the fact, but this is not necessary to qualify as a bonk. Likewise, the most common implement is a baseball bat, but it can be any object or even bare-handed.
See also
>_<: common facial expression of the recipient of a bonk
