What defines the distance a mass moves in a specified direction over a unit of time?

Velocity is defined as the distance a mass moves in a specified direction over a unit of time. It is a vector quantity, which means it not only considers the speed of the mass but also the direction in which it is moving. This distinction is important because two objects can have the same speed but different velocities if they are moving in different directions.

In contrast, while speed measures how fast an object is moving regardless of direction, it does not convey any directional information, which is why it is categorized as a scalar quantity. Acceleration refers to the rate at which velocity changes over time, which provides insight into how the speed or direction of an object is increasing or decreasing. Force involves the influence that causes an object to undergo changes in speed, direction, or shape, but does not define distance moved directly.

Thus, in the context of the question, velocity encapsulates both the distance moved and the specific direction, making it the correct choice.