Protocol
Rules for how two parties should communicate — message format, ordering, error handling, state transitions. TCP, HTTP, SMTP, BGP, OAuth are all protocols.
Internet protocols are standardized in RFCs (Request For Comments) via the IETF. The layered model (OSI 7-layer or TCP/IP 4-layer) abstracts — each protocol sits on a layer and doesn't need to care about the ones below. Application protocols (HTTP, SMTP, MQTT), transport (TCP/UDP), network (IP), link (Ethernet/Wi-Fi). State machines are the most common specification approach. Protocol Buffers (Google) is a different kind of "protocol" — a data contract, not communication.