What is a single phase supply?

is the measured voltage,generated between a line voltage and the starting point of a three phase transformer, which is the neutral point.

Answer: it is an ac circuit in which the supply has a live and neutral wire, usually at one of the standard voltages 110/120v or 220/240 v. A three-phase circuit is a combination of three single-phase circuits with the phases of the voltages 120 degrees apart so that they peak in a regular sequence.