A microlight is an aeroplane weighing less than 450kg on take off.   If an aeroplane weighs more than 450kg it is in the light aircraft category with different procedures and licensing requirements.    So the same aircraft could be either a microlight or a light aircraft dependent on the weight of the engine or it’s instrumentation.   A microlight [known internationally as an ultralight], is restricted to 2 persons and needs to be able to fly slowly as well.

More detail here:BMAA definition.