Here's why:

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

* Deceleration specifically refers to a decrease in the *magnitude* of velocity, which is the speed.

* While deceleration implies a slowing down, it doesn't necessarily mean a change in direction. You can decelerate while maintaining a constant direction (like slowing down a car on a straight road).

It's important to note that deceleration is often used interchangeably with the term "negative acceleration". While they often describe the same phenomenon, it's technically more accurate to say "deceleration" when referring to a decrease in speed.