Deceleration is simply a special case of acceleration where the acceleration is in the opposite direction of the object's velocity. This causes the object's speed to decrease.

Here's why it's still considered acceleration:

* Acceleration is a change in velocity. This change can be an increase in speed (positive acceleration) or a decrease in speed (negative acceleration, or deceleration).

* Velocity has both speed and direction. So, even if the object is moving in a straight line, a change in its direction would also be considered acceleration.

Therefore, deceleration is a specific type of acceleration that results in an object slowing down.