叛国，在法律上，指一个人对其所属于的国家不忠诚；违背其效忠宣言或与其国家的敌人合作的人会被称为叛徒。《法律词典》（Oran's Dictionary of the Law）（1983年版）对叛国的定义是：「一个公民协助外国政府推翻或严重侵害他所属的国家，或怂恿外国对他所属的国家宣战。」

站在本国执政者（尤属长期执政的独裁者）的立场上来看，叛国的相似词是革命，因为革命几乎会利用国内外一切可用势力行来从事推翻原政府政权的行为，叛国与革命的认定则由观点立场的不同而有差异，进行革命的人通常会被原始政府视为叛国或密谋颠覆等，被捕后通常会以前述之罪名予以处决、监禁或是驱逐出境（有外国势力介入者的可能性较大）。但是也有观点认为二者可从受支持的普遍程度上来划分。

A quisling (/ˈkwɪzlɪŋ/; Norwegian pronunciation: [ˈkʋɪsˈlɪŋ]) in Norwegian, English, and some other languages, is a person who collaborates with an enemy occupying force. The word originates from the Norwegian war-time leader Vidkun Quisling, who headed a domestic Nazi collaborationist regime during the Second World War. The word has remained current in various media since it originated through the early 21st century, although the related verb created by back-formation, quisle, has not.