What does constant acceleration mean?

Acceleration:

1. The acceleration of a particle is the rate at which its velocity changes with respect to time.
2. If the particle's velocity changes at a constant rate, this rate is referred to as the constant acceleration.
3. Constant acceleration is defined as a change in velocity that does not vary over time.
4. If a car increases its velocity by 20 mph in one minute, then another 20 mph the next, its average acceleration remains constant at 20 mph per minute.