An Application Programming Interface (API) is a kind of coding (or set of codes) that is used to enable connections – or, transactions – between software applications, databases, operating systems, and servers. One side of the transaction (usually known as the client) will send a request to access the data or features of an API-connected application or database. This request is received by the second party’s API (usually known as the server), which then either allows or denies the request based on a predefined set of rules (these are known as the API specifications).