'terror' originates from Latin, specifically the word 'terror', where the verb root 'terrēre' meant 'to frighten'.

'terror' passed into English via Old French/Anglo-Norman and Middle English (e.g. Middle English 'terour') and eventually became the modern English word 'terror'.

Initially, it meant 'a state of great fear' in Latin and early English; over time it retained that core sense but also came to be used for organized or political violence causing fear (e.g. a 'reign of terror').