In this paper we try to present the definition of verbal threat from the viewpoint of linguistics, to make clear the concept threat which belongs to language crime. There are some debates about whether the threat is an illocutionary act or a perlocutionary act within the speech act theory. We claim that the threatening as a speech act belongs to an illocutionary act, because the effects of threatening on audience are not clear. The threatening can be defined by four felicity conditions in the same way as other speech acts, although there are various views on the felicity conditions about threatening. We can apply those conditions to law cases related with threat, and judge those cases by using those conditions. In the law, the judgement depends on if the harm is given notice to audience or not. In comparison, the linguistic judgement presents more reasonable explanation than the legal judgement.