Is diaphragm another word or mirror on a microscope?

The diaphragm is not another word for the mirror on a microscope, it is part of the condenser. The condenser is a component of a microscope that gathers and focuses light from the light source onto the specimen, and the diaphragm is an adjustable opening that controls the amount of light that passes through the condenser. The mirror on the other hand directs the light from the light source into the condenser.