Data hiding is the modern name of an old science, which has its origins in ancient Greece [1]. It was first known as steganography – a name composed of the Greek words “steganos” (“covered”) and “graphia” (“writing”), which was first documented by the German scholar Johannes Trithemius in his work “Steganographie” [2].

Steganography was a science which focused on the theoretical methods and the practical applications of hiding secret information in various kinds of media. The hidden information had to remain transparent to the human user and normal media processing technologies. It could be read only by means of specialized transformations. Classic examples of the usage of steganography are watermarks and security metal threads hidden in banknotes [3], invisible inks [4] or the micro printing ([5], p. 276), which uses writing of extremely small size (< 0.5 mm) appearing as a thin line to normal human eyes.

The advance of modern communication technologies and digital multimedia  has led to a renewed interest in steganography and its formation as a modern science ([6], pp. 6-11). Its name was changed to the more general term data hiding. At this time, data hiding encompasses two major research incentives which have gradually subdivided the science into two main sub-disciplines: the first one bears the ancient name of data hiding - steganography  - and the second one is called digital watermarking ([7], [6], ch. 1).

Data hiding methods can work on different types of multimedia – text, images, audio, video, animations, etc. We focuse on digital images as the host medium of interest. The main reason for this focus lies in the immense popularity of images in the World Wide Web – almost every modern web portal uses them to enhance its presentation to end users. The obtained results can be easily generalized for video streams, as well, because most video formats encode their key frames in digital image formats.

The term host refers to any digital media, in which steganographic and digital watermarking methods hide information. 

References:
[1] Herodotus, Histories, Book 5., 440 B.C.
[2] J. Peterson. (1997) Steganographia (Secret Writing), by Johannes Trithemius. URL: http://www.esotericarchives.com/tritheim/stegano.htm (accessed May 9, 2009).
[3] Deutsche Bundesbank. Bundesbank - Currency - Euro-Banknotes - Security features. URL: http://www.bundesbank.de/bargeld/bargeld_banknoten_sicherheitsmerkmale.e... (accessed May 9, 2009).
[4] Science Kids. Create invisible ink with lemon juice. URL: http://www.sciencekids.co.nz/experiments/invisibleink.html (accessed May 9, 2009).
[5] H. Trimm, Forensics the easy way, 1st ed.: Barron’s educational series, 2005.
[6] I. J. Cox, M. Miller, J. Bloom, J. Fridrich, and T. Kalker, Digital Watermarking and Steganography, 2nd ed.: Morgan Kaufmann Publishers, 2008.
[7] E. Lin and J. Delp, "A Review of Data Hiding in Digital Images," in Proceedings of the Image Processing, Image Quality, Image Capture Systems Conference (PICS '99), Savannah, Georgia, 1999, pp. 274-278.