Overview
Markdown is a lightweight markup language for creating formatted text using a plain-text editor. John Gruber created Markdown in 2004 as a markup language that is 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.
History
Markdown was inspired by pre-existing conventions for marking up plain text in email and usenet posts, such as the earlier markup languages setext (c. 1992), Textile (c. 2002), and reStructuredText (c. 2002).
In 2002 Aaron Swartz created atx and referred to it as "the true structured text format". Gruber created the Markdown language in 2004, with Swartz acting as beta tester, with the goal of enabling people "to write using an easy-to-read and easy-to-write plain text format, optionally convert it to structurally valid XHTML (or HTML)."
From Wikipedia (CC BY-SA 4.0).