In the bicycle world, especially the BMX and Freestyle realm, the term «mag wheel» is used to refer to moulded fiberglass or plastic wheels, typically with 3−6 moulded «spokes» as opposed to traditional
In the early days of BMX there were metal «mags» made of pot metal, such as «Lester Mags».
As far as I know, nobody has ever made bicycle wheels out of magnesium. The term «mag wheel» is just a slang term for any type of moulded
«Mag» wheels are heavier than spoked wheels, and the rims can't generally handle high tire pressures, but the wheels themselves are very strong and resilient, good for abusive riding applications.