stegano- (Greek: covered, to cover).

1. Defined in dictionaries as the art of writing in ciphers or characters that are not intelligible, except to the persons who correspond with each other; cryptography (secret writing).

2. The art and science of communicating in a way that hides the existence of the communication. Steganography attempts to hide messages inside other harmless messages in such a way that third parties cannot even detect the existence of the concealed message.

3. The science of communicating in a way that hides the existence of the actual communication. The practice of hiding information in a wider bandwidth carrier. This field covers the techniques used in digital watermarking schemes.

4. In cryptography, steganography refers to not only obfuscating (encrypting) data, but hiding the fact that it even exists.

5. The art and science of hiding information by embedding messages within other, seemingly harmless messages.

Steganography works by replacing bits of useless or unused data in regular computer files (such as graphics, sound, text, HTML, or even floppy disks ) with bits of different, invisible information. This hidden information can be plain text, cipher text, or even images.

Unlike encryption, steganography cannot be detected. Therefore, it is used when encryption is not permitted; or, more commonly, steganography is used to supplement encryption. An encrypted file may still hide information using steganography, so even if the encrypted file is deciphered, the hidden message is not seen.

Steganography (literally, "covered writing") dates back to ancient Greece, where some writers say common practices consisted of etching messages in wooden tablets and covering them with wax, and/or tattooing a shaved messenger's head, letting his hair grow back, then shaving it again when he arrived at his contact point.

As stated earlier, some dictionaries say steganography is synonymous with cryptography; however, those in the spy business make a careful distinction between the techniques of cryptography and steganography, although both are aspects of the art of secret communication.

Cryptography is the technique of scrambling a message in a systematic way so that (hopefully) it can be read only by its intended recipient. Steganography, on the other hand, keeps the message secret by hiding the fact that it exists at all. So the microdot of the Cold War spy novels —in which a document is photographically reduced to the size of a pinhead and stuck to an otherwise innocuous typescript or under a postage stamp—is an example of steganography. Invisible ink is another example. There is no doubt that some cryptographers/steganographers combine the two techniques.