API (Application Programming Interface)

An API (Application Programming Interface) is a defined set of rules and contracts that allows different parts of a software system to communicate with each other.

At a high level, an API specifies how data is requested, sent, and interpreted between components, whether within a single codebase or across separate systems. Internally, APIs are often used to structure software boundaries and interactions between modules, ideally keeping interfaces simple, explicit, and aligned with how the system actually behaves rather than adding unnecessary abstraction.

Externally, APIs, such as REST APIs, provide standardized ways for independent systems to interact over a network, typically using HTTP and formats like JSON. These APIs prioritize compatibility, stability, and clarity of communication between services, even if that requires more generalization than internal interfaces.

Overall, an API defines the contract between systems or components: what operations are available, what inputs are expected, and what outputs are returned.