Definition of hitherto adverb from the Oxford Advanced American Dictionary

 

hitherto

 adverb
adverb
NAmE//ˈhɪðərˌtu//
 
, NAmE//ˌhɪðərˈtu//
 
(formal)
 
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until now; until the particular time you are talking about a hitherto unknown species of moth Her life hitherto had been devoid of adventure.
See the Oxford Advanced Learner's Dictionary entry: hitherto