What is a 1 page website called?

It is typically called a single-page website or a one-page site. In web-app terms, you will also hear SPA (single-page application), but not every one-page marketing site is an SPA. A single-page site works well when the message is focused and the goal is simple (one offer, one CTA). If you need multiple services, locations, or SEO topics, multi-page structure is usually better because each page can target a specific intent and keyword set.