API

Application Programming Interface: a contract defining how two pieces of software talk to each other. The waiter between your frontend and your backend.

API is a foundational concept that sits in the Core Fundamentals area of system design. Engineers reach for it whenever they need to reason about real-world trade-offs in that space — not just for textbook correctness, but because real production systems at companies like Netflix, Amazon, and Google make these decisions every day.

If you want to go deeper than this definition — with diagrams, code, and a quiz to lock it in — work through the "API" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.