Texinfo

Texinfo is a documentation system that uses a single source file to produce both online information and printed output. This means that instead of writing two different documents, one for the online help or other online information and the other for a typeset manual or other printed work, you need write only one document. When the work is revised, you need revise only one document. You can read the online information, known as an "Info file", with an Info documentation-reading program. By convention, Texinfo source file names end with a `.texi' or `.texinfo' extension. (From the GNU Project page.)