Various definitions exist. In general, medium voltage is a term used by the electrical power distribution industry. Here are a few ways medium voltage is defined: 

ANSI/IEEE 1585-2002 refers to: Medium Voltage (1 - 35 kV). [It is assumed that this is ac.]

IEEE Std 1623-2004 refers to: Devices rated to medium voltage (1kV - 35 kV). [It is assumed that this is ac.]

NECA/NEMA 600-2003 refers to "medium voltage cables rated from 600 volts to 69,000 volts AC"

Based on many years of experience, WORTAI believes that the voltage range of electrical power distribution products is 1kV-38kV, it covers most of the definitions. Thus, probably a good working definition of medium voltage is from 1kVac to 38kVac.