Assassins (Persian: حشاشين Ḥashshāshīn or باطنیان Bāteniān, Arabic: حشاشين Ḥashshāshīn, Modern Turkish: Haşhaşiler, also Hashishin, Hassassin, or Hashashiyyin) is a misnomer for the Nizari Ismailis applied abusively to them by the Mustali Ismailis during the fall of the decaying Ismaili Fatimid Empire when the two streams separated from each other. In 1122 the Mustalian dynasty Fatimid caliph al-Amir referred to the Nizaris separated from them and "now firmly established in Persia and Syria", abusively as the hashishiyya "without any explanation" and "without actually accusing them of using hashish, a product of hemp".
The term hashishiyya or hashishi as used by Muslim sources is used metaphorically in its abusive sense (i.e. "irreligious social outcasts", "low-class rabble", etc.). "The literal interpretation of this term in referring to the Nizaris (as hashish consuming intoxicated assassins) is rooted in the fantasies of medieval Westerners and their imaginative ignorance of Islam and the Ismailis." These supposedly medieval fantasies were still in vogue as late as 1990 until the publication of the ground breaking book on the Ismailis by the Twelver Shia author and researcher in original documents in Arabic and Persian that had been discovered in the late 20th century concerning the Ismailis in general and the Nizari Ismailis in particular.
In time the Nizari Ismailis of Persia and Syria began to pose a strong military threat to Sunni Seljuq authority within the Persian territories by capturing and occupying many mountain fortresses under their first leader Hassan-i Sabbah (or Hassan bin Sabbah).
The term "assassin" is commonly used to describe a hired killer or a cutthroat. The term later paved the way for another term "assassination" which denotes any action involving murder of a target for political reasons.