A character being hit over the head in a comedic manner and/or accompanied by the eponymous sound effect. Sometimes resulting in a dented head.
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
