What is "line officer"

n. a commissioned officer with combat units (not a staff officer or a supply officer)

In the United States Armed Forces, the term line officer or officer of the line refers to a U.S. Navy, U.S. Marine, U.S. Air Force or U.S. Coast Guard commissioned officer who exercises general command authority and is eligible for operational command positions, as opposed to officers who normally exercise authority within a specialty. The term is not generally used by officers of the U.S. Army—the roughly corresponding Army terms are basic branch and special branch although the concepts are not synonymous, as some special branch Army officers are eligible to hold command.

Officers who are not line officers are those whose primary duties are in non-combat specialties including chaplains, attorneys (only U.S. Air Force and U.S. Navy), supply and medical services. A line officer may hold authority over a non-line officer of higher rank by the nature of their job, but is otherwise expected to observe normal customs and courtesies outside that role.

See explanation of staff and line.