Definition
Markdown is a lightweight markup language that expresses headings, lists, links, quotations and other structures with easy-to-read plain-text symbols such as #, * and >. Written text can then be converted to HTML and other formats.
How it works
Authors write plain text; a renderer parses the markers and produces HTML or another output format. Because the original syntax drifted into dialects across tools, the CommonMark Spec defines consistent parsing behavior, while the Daring Fireball syntax documentation remains the original reference.
Usage
GitHub READMEs, technical documentation, blogs and static site generators (such as Astro and Hugo) all rely on Markdown. Being plain text, it also works well with version control and cross-platform collaboration.
Related and confusable terms
HTML describes web page structure directly, while Markdown prioritizes readability and is typically converted to HTML. JSON carries machine-parsed structured data and is not meant for formatting prose.