The Collaborative International Dictionary of English v.0.48:

follow-up \follow-up\ n.
   1. a second (or subsequent) action to increase the
      effectiveness of an initial action. Also used
      attributively; as a follow-up visit.

   Note: A follow-up may be of various types. After a medical
         examination, a second examination (or reexamination)
         to obtain additional information regarding some fact
         discovered in the first examination is considered a
         follow-up. A second visit or phone call in pursuit of a
         sale or other request would also be a follow-up.

   Syn: reexamination, review.
        [WordNet 1.5 +PJC]

   2. (Journalism) A subsequent story providing information
      discovered or events happening after a first story was
      published.
      [PJC]

   3. (Journalism) Same as sidebar.
      [PJC]

WordNet (r) 3.0 (2006):

follow-up
    n 1: a piece of work that exploits or builds on earlier work;
         "his new software is a follow-up to the programs they
         started with" [syn: follow-up, followup]
    2: an activity that continues something that has already begun
       or that repeats something that has already been done [syn:
       follow-up, followup]
    3: a subsequent examination of a patient for the purpose of
       monitoring earlier treatment [syn: follow-up, followup,
       reexamination, review]