In Short: .md Means Markdown Document

.md is the file extension for Markdown (derived from "Markdown", sometimes also written as .markdown). Markdown is a plain-text format that uses simple symbols to represent formatting, invented in 2004 by John Gruber with the goal of "making writing documents as simple as writing plain text, while automatically producing beautiful formatting."

How simple are the rules? A few examples will make it clear:

  • Start a line with # → it's a heading (## is a second-level heading)
  • Wrap text with ** → the text is bold
  • Start a line with - → it's a list item
  • [text](url) → it's a link

An analogy: A .md file is like "a .txt with formatting instructions." A .txt can only hold text; a .md uses symbols to mark "this is a heading, this is bold," and when a rendering tool processes it, you get a beautifully formatted document.

Why Do You Keep Encountering .md Files?

You may not have heard of Markdown, but it's already everywhere:

  • Open-source project documentation: Nearly every project on GitHub has a README.md;
  • AI responses: ChatGPT, DeepSeek, and Claude generate content in Markdown format by default—saving it gives you a .md file;
  • Technical docs and notes: Programmers and writers use it for documentation and note-taking (note-taking apps like Typora and Obsidian are built around it);
  • Blog and article drafts: Many authors write in Markdown first, then format and publish.

The reason for its popularity is simple: it's plain text, works with any software, and will always be openable. Unlike Word's .docx which depends on Office, a .md file can still be read with any text tool ten years from now—this is also one of the reasons why AI loves Markdown.

How Do You Open .md Files?

Since it's plain text, Notepad can open .md files—but what you'll see is the "source code" with # and * symbols, not the formatted result. To read it properly, you need a tool that can "render" it:

  • Windows / Android: Install mdview, then double-click .md files to read them like web pages—instant open, zero configuration;
  • Quick one-off viewing: Paste content into an online Markdown preview website (be careful with privacy-sensitive documents);
  • For editing: Use editors like VS Code or Typora.

For a detailed comparison of different methods, see How to Open .md Files? The Simplest Methods for Windows and Android; if your .md file shows only symbols and you think it's garbled, check out Why Does My .md File Show Only # Symbols?

Summary

  • .md = Markdown document, a plain-text format that "uses symbols for formatting";
  • It's everywhere in open-source projects, AI responses, and technical notes;
  • Notepad opens it but shows only symbols—use a reader tool like mdview to see the formatted output.

Want to learn more about Markdown itself? This 3-minute intro: What is Markdown goes into more detail.