Metal Halide (MH) or High Intensity Discharge (HID) is a form of lighting that is widely used as indoor grow lights for hydroponic growers, domestic to commercial. With the tech going back as far as the 1960's.
MH lamps emit light when an arc of electricity is sparked between two electrodes inside a quartz tube filled with a noble gas.
MH lights contain metal halide. This gas burns with a cool white light that contains a large amount of light in the blue spectrum.
MH lights are made very similarly to Mercury Vapor Lamps with the difference that they contain additional metal halide compounds in the quartz arc tube, in order to improve efficiency and colour rendition (CRI). They are different.
A CMH grow light is like a Metal Halide (MH) but while a MH bulb uses a standard quartz arc tube, CMH is built with a ceramic arc tube. This is similar to the material used for HPS grow lights.
All HID lamps have a limited life, as the gasses are highly reactive, it leads to a loss of gas over time which will eventually result in the lamp perishing, needing to be replaced.
The expected effective life span of a MH light is around 3000-5000 hours, after that time-frame they will have lost significant lumen output.