protocol
A fixed set of rules that all participants in a network must follow. Bitcoin's protocol defines everything from block size and transaction structure to the 21 million supply cap and halving schedule, with no single party able to change these rules unilaterally.
A protocol is a set of rules that defines how participants in a system communicate and operate. These rules are not suggestions; every participant must follow them or be rejected by the network. The internet, for example, runs on TCP/IP, a protocol that governs how data packets are sent and received across the globe. Without a common protocol, devices could not communicate with each other reliably.
Bitcoin's protocol defines the precise rules of the monetary system: the maximum supply is capped at 21 million bitcoin, a new block is added approximately every ten minutes, the block reward is halved roughly every four years, and transaction formats follow strict cryptographic standards. These rules are enforced by every node running Bitcoin software. No company, government, or founding individual can change them without consensus from the entire network.
This is why protocol-level rules are the foundation of trustlessness in Bitcoin. There is no authority to bribe, threaten, or petition. The rules run as code on thousands of independent machines worldwide. When users say "Bitcoin has a fixed supply," they mean a rule written into the protocol that the entire network enforces automatically and autonomously.