According to MuleSoft's foundational principles of system integration, an interface is the contract that defines the rules of engagement between two or more systems. This contract explicitly specifies the communication methods (e.g., request-response), data formats (e.g., JSON, XML), and protocols (e.g., HTTP, JMS) to be used. In modern integration, an Application Programming Interface (API) serves as this formal interface, providing a standardized way for different software components to interact without needing to know the details of each other's internal implementation.

B. Message: A message is the data payload being exchanged between systems, not the contract that defines the rules for the exchange.

C. Interaction: An interaction refers to the actual act or event of communication, which is governed by the rules defined in the interface.

D. Component: A component is one of the individual systems or applications (e.g., a database, a service) involved in the integration.

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1. MuleSoft Official Documentation, "About APIs": In the introduction, the documentation states, "An API is an interface that makes it easy for one application to “talk to” another... The API is a contract that defines the kinds of requests that a client can make of a service, and the kinds of responses that the service can return." This directly aligns the term "interface" with the definition of a communication contract specifying methods, formats, and protocols.

2. MuleSoft Official Documentation, "API-led Connectivity": This core concept paper describes how APIs function as modern interfaces that abstract underlying system complexity. The API specification (e.g., RAML or OAS) serves as the definitive contract for the interface, detailing resources, methods, data types (formats), and protocols. (See the "What is API-led Connectivity?" section).