The Collaborative International Dictionary of English v.0.48:

Steganography \Steg`a*nog"ra*phy\
   (st[e^]g`[.a]*n[o^]g"r[.a]*f[y^]), n. [Gr. stegano`s covered
   (fr. ste`gein to cover closely) + -graphy.]
   1. The art of writing in cipher, or in characters which are
      not intelligible except to persons who have the key;
      cryptography.
      [1913 Webster]

   2. Specifically: The branch of cryptography in which messages
      are hidden inside other messages; -- used commonly for the
      process of hiding messages inside a computerized image
      file, as for example hiding the name and copyright notice
      of the owner of an image as protection against violation
      of the copyright.
      [PJC]

WordNet (r) 3.0 (2006):

steganography
    n 1: act of writing in code or cipher [syn: cryptography,
         coding, secret writing, steganography]

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

steganography

    Hiding a secret message within a larger one in such
   a way that others can not discern the presence or contents of
   the hidden message.  For example, a message might be hidden
   within an image by changing the least significant bits to
   be the message bits.

   [Chaffing and Winnowing: Confidentiality without Encryption,
   Ronald L. Rivest, MIT Lab for Computer Science, 1998-03-22
   (http://theory.lcs.mit.edu/~rivest/chaffing.txt)].

   (1998-07-13)