Accompanying artillery.—Single batteries, platoons, or pieces attached to assault infantry regiments or battalions for their close support.

Action.—An engagement or battle, usually one on a small scale.

Advance by bounds.—An advance controlled by the assignment of successive movement objectives usually from one terrain line to the next.

Advance by echelon.—An advance of a unit by successive movements of its component elements.

Advance guard.—A security detachment which precedes the main body on the march.

Advance party.—A detachment that is sent out by and moves ahead of the support of the advance guard and forms a reconnoitering element of the support. A detachment which precedes its unit to make administrative or other arrangements.

Aerial (or air) observation.—Observation from balloons, airplanes, or airships.

Air-borne troops.—A general term used to include both parachute and air landing troops.

Alert.—A state of readiness for movement or action. An alarm warning. Vigilant.

Alternate firing position.—A firing position from which the same fire missions can be executed as from the primary firing position.

Ambush.—A concealed place or station where troops lie hidden for the purpose of attacking by surprise. Troops posted in such a position. To attack from such a position.

Approach march.—The advance, usually in extended dispositions from the point where hostile medium artillery fire is expected or air attack is encountered to the point of effective hostile small-arms fire. It ordinarily commences with the development of companies and larger units and terminates with their complete or partial deployment as skirmishers.

Assembly area (or position).—The area in which elements of a command are organized preparatory to further action. For example, in the attack, liaison with supporting arms is arranged; objectives and other missions are assigned to component units.

Axial road.—A road along the contemplated direction of advance.

Bivouac.—An area in which troops rest on the ground with no overhead cover or under natural cover, shelter tents, or improvised shelter.

Bridgehead.—Position occupied by advance troops to protect the passage of a river or defile by the remainder of the command.

Canalize.—To restrict an advance by natural or artificial obstacles and by fire into a narrow zone.

Clear (in the).—The sending of messages, orders, or instructions in plain (uncoded) language.

Clear (verb).—To pass a designated point or line. Refers to the tail of a unit.

Combat echelon—The principal element of offensive or defensive power, for example, the infantry echelon in defense occupying the principal battle position.

Combat vehicle.—A self-propelled, armed vehicle, with or without armor, manned by combat personnel.

Compartments of terrain.—An area of terrain enclosed on at least two sides by critical terrain features such as ridge lines, woods, or water courses.

Convoy—Any group of transportation temporarily organized to operate as a unit during movement. To escort. To accompany for the purpose of protecting.

Corridor.—A compartment of terrain of which the longer dimension lies generally in the direction of movement or leads toward an objective. For example, an avenue of approach having natural terrain features on its two flanks which limit observation and direct fire from positions outside the corridor constitutes favorable lines of advance for friendly or hostile forces.

Counterattack.—An attack by part or all of the defending force against a hostile attacking force for the purpose of regaining ground lost or for destroying hostile elements.

Counterreconnaissance.—Measures taken to screen a command from hostile ground and air observation and reconnaissance.

Cover.—Natural or artificial shelter or protection from fire or observation, or any object affording such protection. The vertical relief of a trench measured from the bottom, or from the trench board, to the top of the parapet. To protect or provide security for another force or a locality.

Defiladed.—Protected from hostile ground observation and fire by a mask.

Delaying action.—A form of defensive action employed to slow up the enemy's advance and gain time without becoming decisively engaged.

Development.—The distribution of a command from mass or route-column dispositions into smaller columns or groups in preparation for action. The extension in width and depth of companies and larger units preparatory to approach march.

Direct fire.—Fire in which the firer aims the weapon by means of sights directly at the target.

Displacement.—The movement of supporting weapons from one firing position area to another, for example, in attack the successive movement of supporting weapons to correspond with the progress of the attacking echelon in order to keep weapons within efficient supporting distance thereof.

Element.—One of the subdivisions of a command. The term "elements" is used in an inclusive sense to refer to all those various smaller units or parts of units, generally different in character; as service elements, meaning quartermaster, ordnance, engineer, and medical units, etc.

Emplacement.—A prepared position from which a unit or a weapon executes its fire missions.

Enfilade (verb).—To fire at a target so that the fire coincides with the long axis of the target, for example, to fire against troops disposed in a generally linear formation from their direct flank and along the direction of their front.

Envelopment.—An attack against one or both hostile flanks, usually assisted by a secondary attack against the enemy's front.

Flank.—The side of a command from the leading to the rear- most element, inclusive. Right flank is the right side when facing the enemy, and does not change when the command is moving to the rear.

Holding attack (secondary attack).—The part of the attack designed to hold the enemy in position and prevent the redistribution of his reserves; it is ordinarily directed against the hostile front.

Hull defilade (or hull down).—Position taken by destroyer which provides defilade for the hull of the vehicle.

Infiltrate.—To pass troops in relatively small numbers through gaps in the enemy position or his field of fire, for example, to advance individuals by bounds during an attack.

Liaison.—The connection established between units or elements by a representative, usually an officer, of one unit who visits or remains with another unit.

Lightly armored vehicle.—An armored vehicle which provides armor protection from small arms fire for its crew and/or engine.

Local security.—A security element, independent or an outpost, established by a subordinate commander to protect his unit against surprise and to insure its readiness for action.

Logistics.—That branch of military art that comprises everything relating to movement, supply, and evacuation.

Main attack.—That part of the attack where the commander concentrates the greatest possible power. Compare Holding attack.

March unit.—A subdivision of a marching column which moves and halts at the command or signal of its commander.

Mask (obstruction).—Any natural or artificial obstruction which interferes with view of fire, usually an intervening hill, woods, etc. Friendly troops located between a gun and its target may constitute a mask.

Motorized unit—A unit equipped either organically or temporarily with sufficient motor vehicles to carry all its matériel and personnel at the same time.

Mutual support.—The support involving fire or movement or both, rendered one another by adjacent elements.

Normal impact.—The impact of a projectile against a surface in which the line of flight of the projectile is perpendicular to the surface struck.

Organization. (See Unit.)

Park.—An area used for the purpose of servicing, maintaining, and parking vehicles.

Penetration—A form of attack in which the main attack seeks to break the continuity of the enemy's front and to envelop the flanks thus created.

Phase line.—A line or terrain feature on which units may be halted for control, coordination, or further orders.

Point.—The patrol or reconnaissance element which precedes the advance party of an advance guard, or follows the rear party of a rear guard.

Position in readiness—A position assumed as a temporary expedient in a situation so clouded with uncertainty that positive action is considered unwarranted.

Rate of march.—The average speed over a period of time including short periodic halts.

Rear guard.—A security detachment which follows the main body and protects it on the march.

Rear party.—The detachment from the support of a rear guard which follows and protects it on the march.

Reconnaissance.—The operation of searching for information in the field.

Reserve.—A fraction of a command held initially under the control of the commander to influence future action.

Retreat.—An involuntary retrograde movement forced on a command as a result of an unsuccessful operation or combat. The act of retreating. To retire from any position or place. To withdraw.

Retrograde movement.—A movement to the rear.

Road block.—A barrier to block or limit the movement of hostile vehicles along a road.

Road space.—The distance from head to tail of a column when it is in a prescribed formation on a road.

Screen.—To prevent hostile ground reconnaissance or observation. The body of troops used to screen a command.

Screening smoke.—A chemical agent used to blind hostile observation.

Sector.—The defense area designated by boundaries within which a unit operates on the defense.

Signal operation instructions (SOI).—A type of combat orders issued for the technical control and coordination of signal agencies throughout the command.

Slit trench.—A very narrow trench used for protection against shell fire and passage of tanks, especially in massing troops close to the front.

Speed.—The rapidity of movement at any particular instant expressed in miles per hour.

Standing operating procedure (SOP).—Routine procedure prescribed to be carried out in the absence of orders to the contrary.

Support (of advance guard).—The echelon of the advance guard that precedes the advance-guard reserve. The support sends out, and is preceded by, the advance party.

Supporting fire.—Fire delivered by auxiliary weapons on the immediate objectives of attacking elements.

Surveillance.—An active, thorough, and continuous search by observation and reconnaissance of an area or of hostile dispositions.

Task force.—A temporary tactical unit, composed of elements of one or more arms and services, formed for the execution of a specific mission or operation.

Unit—A military force having a prescribed organization.

Unit of fire.—The quantity in rounds or tons of ammunition, bombs, grenades, and pyrotechnics which a designated organization or weapon may be expected to expend on the average in one day of combat.

Warning order.—An order issued as a preliminary to another order, especially for a movement, which is to follow; it may be a message or a field order, and may be either written, dictated, or oral. The purpose is to give advance information so that the commanders may make necessary arrangements to facilitate the execution of the subsequent field order.

Wave.—One of a series of lines of forages, mechanized vehicles, skirmishers, or small columns into which an attack unit is deployed in depth.

Zone of action.—A zone designated by boundaries in an advance or a retrograde movement within which the unit operates.

NOTE.—Additional military terms applicable to operations of armored forces are contained in FM 17-10 and other manuals.