Here's why:

* Deceleration is a specific term used to describe a decrease in velocity.

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

* Magnitude refers to the speed of an object.

Therefore, a decrease in both magnitude (speed) and velocity implies that the object is slowing down.

It's important to note that deceleration doesn't necessarily mean an object is slowing down in the conventional sense. If the object is moving in a negative direction and its speed increases, it's still considered deceleration even though the speed is increasing.