Definition: A line manager, sometimes called a direct manager, is a person who directly manages other employees and is responsible for the administrative management of the individual as well as the functional management. If someone refers to their "boss" they mean the individual who is their line manager.
Some managers, like project managers, are responsible for directing the work of other employees, but are not responsible for the administrative management of those individuals. They do not discipline the employee, promote/demote them, do salary adjustments, etc.