You're describing acceleration, which is the rate of change of velocity over time.

Here's a breakdown:

* Velocity: Describes how fast an object is moving and in what direction. It's a vector quantity (meaning it has both magnitude and direction).

* Acceleration: Describes how quickly the velocity of an object is changing. It's also a vector quantity.

Think of it like this:

* If you're driving a car at a constant speed of 60 mph, your velocity is constant, and your acceleration is zero.

* If you press the gas pedal, your car speeds up, meaning your velocity is increasing. This change in velocity over time is your acceleration.

* If you brake, your car slows down, meaning your velocity is decreasing. This change in velocity over time is also acceleration (in this case, negative acceleration, sometimes called deceleration).

Let me know if you'd like a more detailed explanation or examples!