API stands for Application Programming Interface. You can think of it as a common language that lets different software systems communicate smoothly, without needing to know how the other is built. More technically, an API is a set of rules and protocols that determine how one application can request data from another.
In simple terms, an API is both a piece of software running on a networked server and a component of programming code. APIs are standards for application data interchange, just as protocols are standards for network data interchange.