Here's why:

* Acceleration is the rate of change of velocity.

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Negative acceleration indicates that the change in velocity is in the opposite direction of the current velocity.

Example:

* A car traveling at 60 mph slams on its brakes and comes to a stop. This is negative acceleration because the car's velocity is decreasing.

Important Note:

It's common to refer to negative acceleration as deceleration. However, "deceleration" technically just means slowing down, and doesn't necessarily imply that the acceleration is in the opposite direction of the velocity.

For example, a car accelerating from rest has a positive acceleration, but it is also decelerating if its speed is increasing at a slower rate.

In summary:

Use "negative acceleration" when you specifically want to indicate that the acceleration is in the opposite direction of the velocity, resulting in a decrease in speed.