WordPress
Full site editing
Also called FSE.
Full site editing is the WordPress feature set that lets you edit headers, footers, templates, and global styles in the block editor instead of the Customizer and PHP files. It needs a block theme to work.
How it is measured
In wp-admin it appears as Appearance > Editor, which opens the Site Editor with Navigation, Styles, Pages, Templates, and Patterns. Edits you make are stored in the database as wp_template, wp_template_part, wp_navigation, and wp_global_styles posts, not as changes to theme files.
To see what you have overridden, open the Templates view; edited ones are marked Customized. WP-CLI can list them with wp post list --post_type=wp_template.
Worked example
A magazine on a block theme wants a wider post template for long reads. The editor opens Site Editor, duplicates the Single Post template, widens the content block to 960 px, and assigns the new template to 14 specific posts.
Two weeks later the developer pushes a revised single.html in the theme folder and nothing changes on the site, because the stored template wp_template with the slug single overrides the file. He has to clear the customization to see his own update.
How it differs
Full site editing is the editing experience, while a block theme is the theme structure that makes it possible. A classic theme can borrow some pieces, such as the Navigation block in a widget area, but it cannot expose templates in the Site Editor. FSE also does not replace page builders' extra features like popups or conditional layouts.
Common errors
Activating a classic theme and searching for the Editor menu. Letting several people edit templates with no change history. Editing a template on production and forgetting the file in git is now stale. Deleting a navigation menu that other templates reference. Assuming Global Styles changes transfer to a new theme.
In practice
Decide who is allowed into the Site Editor and give everyone else the Editor role without edit_theme_options. When a change is design-level, make it in theme files and git instead. Export from the three-dot menu before any large redesign.