What variable is voltage?

Can you please clarify what you mean by "What variable is voltage?" Voltage, by definition, is not a variable but rather a measure of the potential difference in charge between two points in an electric field. It is expressed in units of volts (V) and is a fundamental quantity in electrical circuits and electronics. Are you perhaps asking about how voltage can vary or what factors can affect its value? Or are you seeking a more basic understanding of what voltage represents in electrical systems?