Here's why:

* Magnitude: Acceleration has a numerical value that represents how much the velocity changes over time. For example, 5 meters per second squared (m/s²) indicates a significant change in velocity.

* Direction: Acceleration also has a direction. It describes the direction in which the velocity is changing. For example, a car accelerating forward has a positive acceleration, while a car braking has a negative acceleration (or deceleration).

Therefore, acceleration requires both magnitude and direction to be fully described, making it a vector quantity.