It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.
The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.
We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.