Definition of momentous adjective from the Oxford Advanced Learner's Dictionary

 

momentous

 adjective
adjective
BrE BrE//məˈmentəs//
 
; NAmE NAmE//moʊˈmentəs//
 
 
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very important or serious, especially because there may be important results synonym historic a momentous decision/event/occasion At the same time, momentous events were taking place in Russia.
See the Oxford Advanced American Dictionary entry: momentous