Figure 2.12 A plane decelerates, or slows down, as it comes in for landing in St. Maarten. Its acceleration is opposite in direction to its velocity.
Average Acceleration is the rate at which velocity changes,
where 
is average acceleration, 
is velocity, and 
is time. (The bar over the 
means average acceleration).
Because acceleration is velocity in 
divided by time in 
, the Sl units for acceleration are 
, meters per second squared or meters per second per second, which literally means by how
many meters per second the velocity changes every second.
Recall that velocity is a vector - it has both magnitude and direction. This means that a change in velocity can be a change in magnitude (or speed), but it can also be a change in direction. For example, if a car turns a corner at constant speed, it is accelerating because its direction is changing. The quicker you turn, the greater the acceleration. So there is an acceleration when velocity changes either in magnitude (an increase or decrease in speed) or in direction, or both.
Acceleration is a vector in the same direction as the change in velocity, 
. Since velocity is a vector, it can change either in magnitude or in direction. Acceleration is therefore a change in either speed or direction, or both.
Keep in mind that 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 known as deceleration.
Figure 2.13 A subway train in Sao Paulo, Brazil, decelerates as it comes into a station. It is accelerating in a direction opposite to its direction of motion.
Source: Rice University, https://openstax.org/books/college-physics/pages/2-4-acceleration
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