An API (application programming interface) is a defined access point or set of functions, with a well-defined syntax, that lets one program request functionality or data from another in a predictable way.
What it means
NIST describes an API as a system access point or library function, accessible from application code, that provides well-defined functionality. In plain terms, an API is a contract that lets software talk to other software. APIs underpin cloud automation, integrations, and network-programmability work (for example in DevOps and CCNA Automation topics).