The degree to which the management functionality of the CMS is bound to the delivery functionality of the CMS.
A coupled CMS has management and delivery bound into the same software. Editors will perform their work on the same running system where visitors consume the content. Both editors and visitors are interacting with different "sides" of the same system.
A decoupled CMS has management functionality in one environment and delivery functionality in another environment (or multiple other environments). When content is published from the management environment, it is transmitted to the delivery environment for visitors to consume. Editors and visitors interact with different systems entirely.
A headless CMS is an informal term to mean a CMS which has editorial and management features, but no delivery features. It exposes content via a web service for consumption by other systems.
Whereas a decoupled CMS will usually template content and transmit it to be consumed, a headless CMS waits for systems to retrieve the content as-needed (pull, rather than push). Some headless CMS are designed as-such, while others are traditional CMS purposefully implemented without delivery features ("headless Drupal," for instance).