headless CMS is a content management system that only handles the backend parts of a traditional CMS (storage, editing interface, APIs, ...), with the visitor-facing side handled by a different system.
For an Indieweb site, a combination of file storage and a Micropub server that are used to create posts that are then rendered into a static site by a static site generator could be called a headless CMS.
- Netlify CMS - edits content stored on Github, actual website is rendered by a static site generator
- Strapi - "The most advanced open-source Content Management Framework to build powerful API with no effort."
- Cockpit - "Simple content platform to manage any structured content. A self-hosted headless and api-driven CMS"
- Contentful - A closed source headless cms as a service. Provides an editing interface and an api to retrieve content
- https://jamstack.org for the more generalized idea behind Netlify CMS