1. Change in velocity: Acceleration is the rate at which an object's velocity changes. This change can be in speed (how fast the object is moving) or direction, or both.

2. Vector quantity: Acceleration has both magnitude (how much) and direction. A car speeding up is accelerating in the direction of its motion, while a car turning a corner is accelerating towards the center of the curve even if its speed remains constant.

3. Caused by force: Acceleration is caused by a net force acting on an object. The more force applied, the greater the acceleration. This is described by Newton's Second Law of Motion: Force = Mass x Acceleration.