The mirror is a part in a SLR that diverts light into the viewfinder or other sensors.
In the majority of SLRs, the mirror is part of the optical pathway that diverts light into the viewfinder. In newer Sony DSLRs, a semi-transparent mirror is used. In this case, the light is diverted only into sensors for focusing or metering. These mirrors are stationary. In other SLRs, the mirror flips up to allow light to pass through to the sensor.