Hidden data concealed in otherwise unsuspicious pictures, text, and/or audio.
Steganography is the art of concealing data within some other message that seems ordinary. The word steganography comes from the Ancient Greek words steganos/στεγανός (meaning covered, concealed, or protected), and graphein/γράφειν (meaning writing).
The concealed message can sometimes be plainly visible if you know where to look; other times, it may require some advanced cryptographic techniques or data manipulation. (For instance, a message is commonly hidden in the low order bits of an image, which doesn't make much of a difference to the naked eye.)
The advantage of steganography over cryptography alone is that the concealed message does not attract attention to itself as an object of scrutiny.