What Is The Definition For Voltage?

Voltage is electromotive force, sometimes referred to as EMF.
Voltage is the force that pushes electrons through the conductor.
 
Electrical circuits are sometimes compared to water piping systems.
In that analogy, voltage is the equivalent of the water pump that forces the water through the pipe.
 
This is a gross oversimplification and you can find more detail at Wik.

Voltage, also called electromotive force is the potential difference in charge between two points in an electrical field. The symbol of voltage is V.

Voltage can be direct or alternating. A direct voltage is one which  maintains the polarity same at all times,where as in an alternating voltage, the polarity reverses direction periodically.