Motion occurs anytime an object changes its position. This is entirely dependent on the frame of reference! Motion can be measured by distance, displacement, speed, velocity, and acceleration.
Velocity refers to an object's speed and direction. Acceleration refers to the rate of change in an object's velocity over time. An object can be moving at a constant speed and still accelerating if it is changing direction. When an object speeds up, it has a positive acceleration.
When an object slows down, it has a negative acceleration. Motion graphs can be used to help us visualize an object's motion. On a distance vs. time graph, the slope of the line equals the object's speed, whereas on a velocity vs. time graph, the slope of the line equals the object's acceleration.