When it comes to voltage level there is no clear boundary between medium and high voltage. Your local standards and industry play a major role in specifying clear boundaries of voltage level. Given below are a few standards and general categorizations of Medium Voltage Level
General categorization of Medium Voltage Level: Between 1000 V and 45 kV (Sometimes 1 kV upto 100 kV is referred as medium voltage level however this is rare)
ANSI standards C84.1-1989 Medium Voltage level: Medium Voltage (MV) – 2400 V to 69 kV
NEC System Medium Voltage Level: Medium Distribution levels – 50 to 1000 volts
In the Electrical Power Distribution industry the term medium voltage refers to a voltage range extending from 2.4 kV to 35 kV. Also see Medium voltage circuit breakers.
IEV 601-01-28 Medium Voltage levels: 1 kV upto 52 kV
So that was all about medium voltage level.
The above-provided information is general purpose only and while dealing with Electricity you should always refer to qualified professional and local standards applicable to your specific industry.