Short definition
A headless CMS is a content management system that stores and serves content through APIs instead of rendering it inside a coupled website front end.
In depth
Headless CMS architecture decouples content from presentation. Editors work in a familiar interface (Sanity, Contentful, Payload, Storyblok, Strapi) while developers consume the content via API in any front-end framework (Next.js, Astro, etc.). Benefits include faster sites (static generation), reusable content across web and mobile, and freedom from monolithic platform lock-in. The trade-off is more setup compared to all-in-one platforms.
Example
An agency moves a Wix client to Next.js + Sanity. The site loads twice as fast and the marketing team publishes weekly without developer help.