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"

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "afterthought":
   PS, Parthian shot, about-face, addendum, afterthoughts, appendix,
   back matter, better thoughts, bind, block, blockage,
   bureaucratic delay, change of mind, chorus, coda, codicil,
   colophon, conclusion, consequence, continuance, continuation,
   delay, delayage, delayed reaction, detention, developed thought,
   double take, dragging, dying words, envoi, epilogue, flip,
   flip-flop, follow-through, follow-up, halt, hang-up, hindrance,
   holdup, interim, jam, lag, lagging, last words, logjam,
   mature judgment, mature thought, moratorium, obstruction,
   paperasserie, parting shot, pause, peroration, postface, postfix,
   postlude, postscript, re-examination, reappraisal, reconsideration,
   red tape, red-tapeism, red-tapery, refrain, reprieve, respite,
   retardance, retardation, rethinking, revaluation, reversal,
   reverse, review, right-about, right-about-face, ripe idea,
   second thought, second thoughts, sequel, sequela, sequelae,
   sequelant, sequent, sequitur, slow-up, slowdown, slowness, stay,
   stay of execution, stop, stoppage, subscript, suffix, supplement,
   suspension, swan song, tag, tergiversating, tergiversation, tie-up,
   time lag, turnabout, turnaround, volte-face, wait