In physics, constant velocity occurs when there is no net force acting on the object causing it to accelerate. In terms of airplane flight, the two main forces influencing its velocity forward are drag and thrust.

In aviation physics, an aircraft flying at a constant velocity means it is in a state of dynamic equilibrium, where the four fundamental forces of flight—Lift, Weight, Thrust, and Drag—are perfectly balanced. Velocity is a vector quantity, so "constant velocity" implies that both the speed and the direction of the aircraft are unchanging. For this to occur, the Thrust generated by the engines must exactly equal the total Drag (air resistance), and the Lift generated by the wings must exactly equal the total Weight of the aircraft. When these forces are balanced, the net force acting on the plane is zero, in accordance with Newton’s First Law of Motion. Passengers experience this as a very smooth "stable cruise" where they feel no acceleration or deceleration. From a pilot's perspective, this is the most fuel-efficient phase of flight. Any change—such as a gust of wind increasing drag or a pilot pulling back on the yoke to increase lift—will break this equilibrium, causing an acceleration (or deceleration) until a new state of constant velocity is reached at a different altitude or speed.