Source : Webster's Revised Unabridged Dictionary (1913)

Afterthought \Aft"er*thought`\, n.
   Reflection after an act; later or subsequent thought or
   expedient.

Source : WordNet®

afterthought
     n 1: thinking again about a choice previously made; "he had
          second thoughts about his purchase" [syn: {reconsideration},
           {second thought}, {rethink}]
     2: an addition that was not included in the original plan; "the
        garage was an afterthought"