Final answer:
The term for speed in a chosen direction is 'velocity'. Velocity is a vector quantity, including both speed and direction. Acceleration is the rate at which velocity changes.
Step-by-step explanation:
The term used to describe speed in a chosen direction is velocity. Velocity is a vector quantity because it describes both the magnitude (how fast an object is moving) and the direction of the movement. For instance, stating that a car is moving at 20 kilometers per hour due south provides its velocity, while saying it is moving at 20 kilometers per hour without designating a direction refers to its speed only, which is a scalar quantity.
When discussing the rate at which velocity changes, we are referring to acceleration. Acceleration occurs when there is a change in the magnitude of speed, the direction of movement, or both. For example, a car that turns a corner at a constant speed experiences acceleration, as the direction of its velocity is changing.