A scalar quantity is a physical quantity that has magnitude but no direction. It is completely described by a single number with appropriate units. Air pressure is an example of a scalar quantity because it is a measure of the force exerted per unit area by the air inside the tire. It has a magnitude (the amount of pressure) and no direction (the pressure is exerted equally in all directions).

In contrast, a vector quantity is a physical quantity that has both magnitude and direction. It is completely described by a number, units, and a direction. For example, velocity is a vector quantity because it has magnitude (speed) and direction (the direction in which the object is moving).