Acceleration is a fundamental concept in physics that describes the rate of change of an object's velocity with respect to time. It represents how quickly the velocity of an object is changing, either in magnitude or direction or both. When an object accelerates, its velocity increases, decreases, or changes direction.

Acceleration is a vector quantity, meaning it has both magnitude and direction. The magnitude of acceleration is expressed in units of meters per second squared (m/s²) in the International System of Units (SI). One meter per second squared represents an acceleration of one meter per second increase in velocity every second.