What is the common name of optical device on firearm?

There are a number of optical devices used on firearms. The most common is a telescoping sight, usually referred to as a "scope". There are others however, that while optical, do not magnify the image. These are sometimes referred to as "red dot" sights, and they use a sighting method that seems to suspend a red dot in the air where the bullet impact will be. Another version of the same thing is referred to as a "holographic" sight.