A field officer or field grade officer is an Army, Marine, or air force commissioned officer senior in rank to a company officer but junior to a general officer; in the U.S. Navy, it is an officer who is a Lieutenant Commander, Commander, or Captain.
Today, a field officer is typically a major, lieutenant colonel, or a colonel. The British Army, (in which these officers are called officers of field rank, not field grade) and some others in the Commonwealth (including the Australian Army and New Zealand Army), also include the brigadier in this category: the equivalent in other armies is a general officer (the brigadier general).
In the Canadian Forces, the equivalent of field officers are senior officers (Officiers supérieurs); they include the army and air force ranks of Major, Lieutenant-Colonel, and Colonel, and the naval ranks of Commander, and Captain.
"The [U.S.] commissioned officer corps is divided into 10 pay grades (O-1 through O-10). Officers in pay grades O-1 through O-3 are considered company grade officers. In the Army, Marine Corps, and Air Force, these pay grades correspond to the ranks of second lieutenant (O-1), first lieutenant (O-2), and captain (O-3), and in the Navy, ensign, lieutenant junior grade, and lieutenant. Officers in the next three pay grades (O-4 through O-6) are considered field grade officers. In the Army, Marine Corps, and Air Force, these pay grades correspond to the ranks of major (O-4), lieutenant colonel (O-5), and colonel (O-6), and in the Navy, lieutenant commander, commander, and captain. The highest four pay grades are reserved for general officers in the Army, Marine Corps, and Air Force, and flag officers in the Navy. The ranks associated with each pay grade are as follows: in the Army, Marine Corps, and Air Force, brigadier general (O-7), major general (O-8), lieutenant general (O-9), and general (O-10); in the Navy, rear admiral-lower half, rear admiral-upper half, vice admiral, and admiral." (See: http://www.defenselink.mil/prhome/poprep2000/html/chapter4/chapter4_2.htm or http://www.defenselink.mil/specials/insignias/officers.html.)
In the German Forces, Field Officers (ranging from Major to Colonel in the Army and Luftwaffe, from Lieutenant Commander to Captain in the Navy) are known as Staff Officers (Stabsoffiziere).