steganography

noun2

1. •
   The art and science of concealing a secret message, data, or file within another innocuous message, image, audio file, or physical object in a way that hides the very existence of the hidden information from casual observation.
2. •
   The use of small computer files to communicate secret information.

   • Using the steganography tools available today, suspects can even hide data inside JPEGs and audio files. — 2005