'afterthought' definitions:

Definition of 'afterthought'

From: WordNet
noun
Thinking again about a choice previously made; "he had second thoughts about his purchase" [syn: reconsideration, second thought, afterthought, rethink]
noun
An addition that was not included in the original plan; "the garage was an afterthought"

Synonyms of 'afterthought'

From: Moby Thesaurus