Watermarking is similar to steganography, but is not quite the same thing. (You might, however, see them discussed together). In the old-fashioned case, both invisible ink and an authenticating watermark might appear on a sheet of paper and require special procedures to reveal. In digital data, almost exactly the same situation exists. But the purpose of a watermark is always to be implicitly available for revelation in appropriate circumstances; the purpose of steganography is to hide its own existence from those unaware of its method of revelation. In digital terms, a watermark might be something a copyright holder puts inside a digital image to prove she is the owner, whereas a steganographic message might be something a political dissident puts inside a digital image to communicate with other dissidents (in which case, a repressive government could not prove the message was sent at all, that it's not simply a family photo). The techniques for concealing the subtext might be similar, but the concealer's relation to an attacker is almost exactly opposite.