A commissioned officer with combat units (not a staff officer or a supply officer) WordNet 3.0 & Open English WordNet

In the United States Armed Forces, a line officer or officer of the line is a U.S. Navy or U.S. Marine Corps commissioned officer or warrant officer who exercises general command authority and is eligible for operational command positions, as opposed to officers who normally exercise command authority only within a Navy Staff Corps. Wikipedia

A commissioned officer who exercises general command authority and is eligible for operational command positions. Wiktionary