Acceleration occurs when velocity changes in magnitude (an increase or decrease in speed), direction, or both. Although acceleration is in the direction of the change in velocity, it is not always in the direction of motion. When an object slows down, its acceleration is opposite to the direction of its motion. This is commonly referred to as deceleration. However, the term deceleration can cause confusion in analysis because it is not a vector; it does not point to a specific direction with respect to a coordinate system, so there should be an appropriate sign for it in the chosen coordinate system. If the acceleration of an object is in the negative direction in the chosen coordinate system, the object is said to be undergoing negative acceleration.

Imagine an object in motion has a velocity in the positive direction with respect to a chosen origin, and it acquires a constant negative acceleration. In that case, the object eventually comes to a rest and reverses direction. After some time, the object passes through the origin, going in the opposite direction.

This text is adapted from Openstax, University Physics Volume 1, Section 4.2: Acceleration Vectors.