In military terminology, a chalk is a specific aircraft load, especially a group of airborne soldiers which deploy from a single aircraft. A chalk often corresponds to a platoon sized unit for air-assault operations, or a company-minus sized organization for airborne operations. For air-transport operations, it can consist of up to a company-plus sized unit.
The term was popularized during the Vietnam War when it was common practice to number with chalk the sides of the helicopters involved in an operation.
The term was first coined in World War II for airborne troops during the Allied invasion of Fortress Europe. The aircraft flight number was placed on the troops' backs with chalk, thus the name Chalk for Chalk 1, Chalk 2, etc.