* Acceleration is a change in velocity. Velocity includes both speed *and* direction.

* Slowing down means a change in speed. Since speed is part of velocity, slowing down means a change in velocity.

* A change in velocity means acceleration.

Therefore, an object that is slowing down is *accelerating*, but in the opposite direction of its motion. This is often called deceleration.

Example: Imagine a car braking to a stop. The car is slowing down, meaning its velocity is decreasing. This decrease in velocity is acceleration in the direction opposite the car's motion.