Steganography is the study of hidden messages.
The term steganography is derived from Greek stego- + graphoi, meaning "plate writing," because it was originally implemented by placing the message to be hidden between two thin wax plates, which were then fused together and a new message written on top. The resulting tablet looked perfectly normal to anyone who might happen to inspect it, but the original message would still be accessible to anyone who knew it was inside.
Steganography hit its heyday during the second World War, when Allied troops in Europe used various means to hide their communications back home; for example, encoding secret messages in the first letter of each word, or using patterns of punctuation to spell out messages in Morse code.
Because it allows messages to "hide in plain sight," Steganography has been used by Al Qaeda to send messages over the internet. (See Jack Kelley, USA Today, "Terrorist instructions hidden online") However, the NSA is also known to use steganographic tools to help with uncovering these plots.