Acceleration means speeding up or moving faster and faster. When a car accelerates, it goes from 20 miles per hour to 40 to 60, gaining speed with each moment. When a roller coaster accelerates down a steep drop, you feel yourself pressed back into your seat as the speed increases.

Scientists use acceleration to describe any change in velocity, which technically includes slowing down (called negative acceleration or deceleration) or even changing direction while keeping the same speed. But in everyday conversation, when someone talks about acceleration, they usually mean speeding up.

You can feel acceleration. When an elevator starts rising, that brief moment where your stomach feels funny? That's acceleration. When a sprinter explodes off the starting blocks, that burst of speed is acceleration. Race car drivers prize quick acceleration because it helps them zoom ahead of competitors.

In physics, acceleration measures how quickly velocity changes over time. A rocket has tremendous acceleration, going from motionless on the launch pad to thousands of miles per hour in minutes. A bicycle has gentle acceleration, gradually building speed as you pedal.

Outside of motion, people sometimes use acceleration metaphorically to describe processes that speed up, like the acceleration of technological change or how learning accelerates once you grasp the basics of a subject.