The Collaborative International Dictionary of English v.0.48:

Cryptography \Cryp*tog"ra*phy\ (-f?), n. [Cf. F. cryptographie.]
   1. The act or art of writing in code or secret characters;
      also, secret characters, codes or ciphers, or messages
      written in a secret code.
      [1913 Webster +PJC]

   2. The science which studies methods for encoding messages so
      that they can be read only by a person who knows the
      secret information required for decoding, called the key;
      it includes cryptanalysis, the science of decoding
      encrypted messages without possessing the proper key, and
      has several other branches; see for example
      steganography.
      [PJC]

WordNet (r) 3.0 (2006):

cryptography
    n 1: the science of analyzing and deciphering codes and ciphers
         and cryptograms [syn: cryptanalysis, cryptanalytics,
         cryptography, cryptology]
    2: act of writing in code or cipher [syn: cryptography,
       coding, secret writing, steganography]

The Free On-line Dictionary of Computing (19 January 2023):

cryptography

    The practise and study of encryption and
   decryption - encoding data so that it can only be decoded by
   specific individuals.  A system for encrypting and decrypting
   data is a cryptosystem.  These usually involve an algorithm
   for combining the original data ("plaintext") with one or
   more "keys" - numbers or strings of characters known only to
   the sender and/or recipient.  The resulting output is known as
   "ciphertext".

   The security of a cryptosystem usually depends on the secrecy
   of (some of) the keys rather than with the supposed secrecy of
   the algorithm.  A strong cryptosystem has a large range of
   possible keys so that it is not possible to just try all
   possible keys (a "brute force" approach).  A strong
   cryptosystem will produce ciphertext which appears random to
   all standard statistical tests.  A strong cryptosystem will
   resist all known previous methods for breaking codes
   ("cryptanalysis").

   See also cryptology, public-key encryption, RSA.

   Usenet newsgroups: news:sci.crypt,
   news:sci.crypt.research.

   FAQ MIT
   (ftp://rtfm.mit.edu/pub/usenet/news.answers/cryptography-faq/).

   Cryptography glossary
   (http://io.com/~ritter/GLOSSARY.HTM#BruteForceAttack).

   RSA cryptography glossary
   (http://rsasecurity.com/rsalabs/faq/glossary.html).

   Cryptography, PGP, and Your Privacy
   (http://draco.centerline.com:8080/~franl/crypto.html).

   (2000-01-16)