High, Low and Extra Low Voltage.
The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage circuits as those with more than 1000 V for alternating current and at least 1500 V for direct current, and distinguish it from low voltage (50 to 1000 V AC or 120 to 1500 V DC) and extra-low voltage (below 50 V AC or below 120 V DC) circuits. This is in the context of building wiring and the safety of electrical apparatus.

Low Voltage.
This is an electrical engineering term that broadly identifies safety considerations of an electricity supply system based on the voltage used. While different definitions exist for the exact voltage range covered by “low voltage“, most usually 50 to 1000 V AC, the most commonly used ones include “mains voltage“, 230 V AC. “Low voltage” is characterised by carrying a substantial risk of electric shock, but only a minor risk of electric arcs through air. “Low voltage” is distinguished from:

Extra low voltage – which carries a much reduced risk of electric shock
High voltage – where electrical arcing is a substantial additional risk.

This should not be mistaken, by the average person on the street, with commonly understood terms of “Low Voltage Lighting” which in electrical engineering terms actually should be “Extra Low Voltage Lighting” for example.

Extra Low Voltage
In electricity supply, the use of extra-low voltage (ELV) is one of several means to protect against electrical shock. The International Electrotechnical Commission and its member organizations define an ELV circuit as one in which the electrical potential of any conductor against earth (ground) is not more than either 25 volts RMS (35 volts peak) for alternating current, or ripple-free 60 volts for direct current under dry conditions. Lower numbers apply in wet conditions, or when large contact areas are exposed to contact with the human body.

High Voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements. High voltage is used in electrical power distribution (overhead high voltage cables and pylons for example), in cathode ray tubes, to generate X-rays and particle beams, to demonstrate arcing, for ignition, in photomultiplier tubes, and in high power amplifier vacuum tubes and other industrial and scientific applications.

An Electric Arc
This is when an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc as used in the phrase “voltaic arc lamp”.