From CryptoDox, The Online Encyclopedia on Cryptography and Information Security
The word "Steganography" is of Greek origin and means "covered, or hidden writing".
Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the meaning is obscured.
There are several situations where steganography can be useful, including:
Steganography is currently used in many color printers, encoding the serial number of the printer on every piece of paper it prints out. This is being used used to track down counterfeiters (people who print out copies of paper currency or train tickets).
An image with a hidden message in it is called a "stego-image".
A text with a hidden message in it is called a "stegotext".
"It is ... an open question whether a computer can alter a natural language text in a way that is undetectable to a human -- that is, [steganographically] embed a ciphertext [into a covertext] ... -- and the problem is commended to the research community as the "Stego Turing Test."" -- Ross Anderson and Fabien Petitcolas in the paper "On the Limits of Steganography" 1998.