1. Change in velocity: Acceleration is the rate of change of velocity. This means it describes how quickly the velocity of an object is changing. If the velocity is increasing, the acceleration is positive. If the velocity is decreasing, the acceleration is negative (also called deceleration).

2. Time: The change in velocity is measured over a specific period of time. The greater the change in velocity over a given time, the greater the acceleration.

In mathematical terms, acceleration is defined as:

Acceleration (a) = Change in Velocity (Δv) / Time (Δt)