API Application Programming Interface
A structured way for two programs to talk to each other.
Think of it as a menu in a restaurant: the calling code says "I'd like pasta number 5", and the kitchen (the API) delivers the pasta. You don't need to know how it's made, just what the menu says.
Common API styles: REST (web-standard), GraphQL (more flexible), gRPC (fast internal communication), WebSocket (real-time).