Markdown is a lightweight markup language for creating formatted text using a plain-text editor. John Gruber created Markdown in 2004, in collaboration with Aaron Swartz, as a markup language that is intended to be easy to read in its source code form. Markdown is widely used for blogging and instant messaging, and also used elsewhere in online forums, collaborative software, documentation pages, and readme files.
The initial description of Markdown contained ambiguities and raised unanswered questions, causing implementations to both intentionally and accidentally diverge from the original version. This was addressed in 2014 when long-standing Markdown contributors released CommonMark, an unambiguous specification and test suite for Markdown.