Duty cycle is expressed as a percentage, put simply it is the percentage of on time within a fixed time period.

 

The duty cycle is defined as the width of the pulses (on time) divided by the period (on time + off time) multiplied by 100%.

 

Example

A signal with a duty cycle of 20% will be high (on) for 20% of its full period and low (off) for 80% of its full period.