Definition of thenceforth adverb from the Oxford Advanced American Dictionary

 

thenceforth

 adverb
adverb
NAmE//ˈðɛnsfɔrθ//
 
, NAmE//ˌðɛnsˈfɔrθ//
 
(also thenceforward
NAmE//ˌðɛnsˈfɔrwərd//
 
)
(old use or formal)
 
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starting from that time Thenceforth he became known as the Lord Protector.
See the Oxford Advanced Learner's Dictionary entry: thenceforth