1. Change in Velocity: This is the most fundamental definition. Acceleration is the rate at which an object's velocity changes over time. This change can be in speed, direction, or both.

2. Change in Speed: This is a simplified definition often used in introductory physics. If an object speeds up, it's accelerating. If it slows down, it's decelerating (which is still considered acceleration, just in the opposite direction of the motion).

3. Change in Direction: Even if an object maintains a constant speed, it can still accelerate if it changes direction. For example, a car going around a curve at a constant speed is still accelerating because its velocity is changing direction.

It's important to remember that acceleration is a vector quantity, meaning it has both magnitude (how much) and direction.