Definition of casus belli noun from the Oxford Advanced Learner's Dictionary

 

casus belli

 noun
noun
BrE BrE//ˌkeɪsəs ˈbelaɪ//
 
; NAmE NAmE//ˌkeɪsəs ˈbelaɪ//
 
; BrE BrE//ˌkɑːsʊs ˈbeliː//
 
; NAmE NAmE//ˌkɑːsʊs ˈbeliː//
 
(pl. casus belli)(formal)
 
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an act or situation that is used to justify a war Word OriginLatin, from casus ‘fall’ and belli, genitive of bellum ‘war’.
See the Oxford Advanced American Dictionary entry: casus belli