API
Also called: application programming interface
The interface your code uses to send a prompt to a model and get a response back.
An API is how software talks to software. For AI it means you post a request containing your prompt and settings and receive generated output, paying per token. It is what makes a model a component you can build on rather than a website you visit.
In practice: One HTTPS request in, one JSON response out, billed by tokens used.