To slow down. To reduce motion. As opposed to accelerate.

From the Dictionary of Technical terms, as used by NASA:

The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.
The driver applied her brakes in order to decelerate and avoid the tree branch which had fallen into the road.
There is no such thing as Deceleration!

Well, not technically.

Objects do not simply "decelerate," and stop, their energy is actively taken from them, and are stopped.

This is known as Negative acceleration.

Negative acceleration is the force applied by all things against an object that take energy from its motion, such as friction, or a large unmoveable object, and either re-apply it (such as the large object), or convert it to heat (friction). As such, all objects opposing this movement apply a negative acceleration to the object.

Say I'm going forward at 10 m/s, to stop me a force must be applied. To stop me in one second, you apply a negative acceleration of -10m/s and I'll come to an instant halt. Unless that force is applied (or any other force of some varying magnitude), I'll just continue on, never stopping.

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