Definition of afterthought noun from the Oxford Advanced Learner's Dictionary

 

afterthought

 noun
noun
BrE BrE//ˈɑːftəθɔːt//
 
; NAmE NAmE//ˈæftərθɔːt//
 
[usually singular]
 
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a thing that is thought of, said or added later, and is often not carefully planned They only invited Jack and Sarah as an afterthought. He said, ‘I won’t tell anyone,’ adding as an afterthought, ‘at least, not if you do me a favour.’
See the Oxford Advanced American Dictionary entry: afterthought