A data API (application programming interface) is a defined set of rules that lets one program request specific data or actions from another without knowing how that system works inside. It's the contract: send this request, get back a predictable, structured response — usually as JSON.
Think of it like ordering at a counter. You (the client) send a request to an endpoint — "give me this product's price and stock." The server validates it, does the work, and returns a structured answer. You never touch the database directly; the API decides what you're allowed to ask for and how the answer is shaped, while an API key or token controls who gets access and how often.
Modern SEO and AI workflows run on APIs. A rank tracker pulls positions through a search-data API, and an AI agent reaches live information through connectors and Model Context Protocol servers, which are themselves wrappers around underlying data APIs. When a language model needs a real fact instead of a guess, a data API is usually what fetches it. That plumbing is central to modern AI SEO.