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):

followup
    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]

The Jargon File (version 4.4.7, 29 Dec 2003):

followup
 n.

    [common] On Usenet, a posting generated in response to another posting
    (as opposed to a reply, which goes by email rather than being broadcast).
    Followups include the ID of the parent message in their headers; smart
    news-readers can use this information to present Usenet news in
    ?conversation? sequence rather than order-of-arrival. See thread.


The Free On-line Dictionary of Computing (30 December 2018):

followup
reply

   On Usenet, a posting generated in response to another
   posting (as opposed to a reply, which goes by e-mail rather
   than being broadcast).  Followups include the ID of the
   parent message in their headers; smart news-readers can use
   this information to present Usenet news in "conversation"
   sequence rather than order-of-arrival.  See thread.

   [Jargon File]