In order to communicate or store information, a mapping between meaning and a sequence or arrangement of observable signals, symbols, events or cues is needed. A language can be considered to be just such a mapping. Communication happens not only between humans, but also between machines and between machines and humans. Even animals, plants, and maybe atoms can communicate. Languages manifest themselves not only as voice and text, but also as body signs, electric impulses and more. Here, mostly formal languages are covered, such as programming languages, markup languages, and protocols.