Open-source software is software whose source code is public. Its licence lets anyone read, use, change and share it. The Open Source Initiative keeps the widely used definition.
Bitcoin is open source
Bitcoin Core, the most used node software, is published under the MIT License (read 23 Oct 2026). Its full history is public on GitHub. Anyone can read every rule a node enforces, suggest changes, or start their own version.
The problem → Bitcoin's answer
Problem: with closed software you must trust the maker about what it does. Answer: with open code, independent reviewers can check it. Bitcoin Core releases also come with signed checksums and reproducible builds, so you can confirm that the file you download matches the public code.
Problem: open code does not mean someone reviews it. Answer: Bitcoin has paid, independent reviewers (funded through grants) and a long record of public review. Still, you only gain the benefit if you verify downloads.
Why it matters
Open code means no company owns Bitcoin. If a team made bad changes, users could refuse to run them.
Takeaway: "Don't trust, verify" only works because the code is open.