Source Code
The human-readable instructions that make up a computer program. Bitcoin's source code defines every rule of the system and is publicly available for anyone to read and verify.
Source code is the text written by programmers that instructs a computer what to do. Unlike a compiled binary that a machine runs directly, source code can be read by people with programming knowledge. It is the human-readable layer that defines the behaviour of software before it is translated into machine instructions.
Bitcoin's source code is open source, meaning it is published publicly on GitHub and accessible to anyone. This is significant: every rule of Bitcoin — the maximum supply of 21 million, the block reward halving, how transactions are validated — exists as code that can be read and verified. There is no need to trust claims about how Bitcoin works; the source code is the definitive and transparent answer.