Duty cycle
The fraction of a period a signal spends HIGH, as a percentage. A 25% duty square wave is on a quarter of the time — and carries a quarter of the power a continuous signal would.
In practice
It's the control variable of almost everything switched: PWM dimming, motor speed, buck converter regulation, the 555's blink ratio. The classic astable 555 can't go below 50% without the extra diode trick — a question every 555 user eventually asks.