A protocol is defined as a set of rules for data processing and formatting. These rules that make up a protocol dictate how data can be shared between computers.
Within the context of cryptocurrencies, a protocol defines the structure of a blockchain network. Examples of blockchain protocols include the Bitcoin protocol and the Ethereum protocol.
Within the blockchain industry, the term protocol is also used to denote decentralized applications, e.g., DeFi platforms. Some examples of DeFi protocols include Uniswap and Aave.