Here's why:

* Acceleration refers to a change in velocity, either increasing or decreasing.

* Deceleration specifically indicates a decrease in velocity.

Think of it like this:

* Acceleration: Stepping on the gas pedal and making your car go faster.

* Deceleration: Stepping on the brake pedal and making your car go slower.

While "negative acceleration" is technically correct, "deceleration" is the more common and understandable term in everyday usage.