The Collaborative International Dictionary of English v.0.48:

Afterthought \Aft"er*thought`\, n.
   1. Reflection after an act; later or subsequent thought.
      [1913 Webster]

   2. an action taken after another action and related to the
      first action, which would normally or optimally be done
      along with the first action; -- as, to do something as an
      afterthought.
      [PJC]

   3. a feature or part added to a device, not thought of in its
      original design.
      [PJC] Afterwards

WordNet (r) 3.0 (2006):

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