Simple definition
A systemd unit defines a managed resource such as a service, socket, timer, mount, or target.
Purpose and use
A unit is systemd’s configuration object for a service, mount, socket, target or another managed resource.
How does it work?
Its suffix and directives determine the type; dependencies and overrides can extend the main file.
Example and verification
Inspect effective configuration, including drop-ins, when behavior differs from the file you examined.
Points to watch
Editing a file does not establish that the manager reloaded it or that a running process uses the new configuration.
In short
Distinguish stored definition, loaded definition and runtime state.