By what other name is the unit joule/coulomb called?

The other name of the unit joule/coulomb is volt.

Volt is the SI unit of electric potential or of electromotive force. It is represented by the letter V. Volt is a representation of the potential difference between the two points of a current-carrying wire of 1 ampere such that when the power dissipates between these two points, it is equal to 1 watt.