All SLR cameras use a mirror and a penta prism
model to reflect light through the camera body and the photographer can see the
scene through the view finder. The below video demonstrates the light path
through a SLR camera.
How SLR camera work
Since the light reflects of the
mirror it gets the term Reflex. Combining the definitions based on how light
enters and reflects its called a Single Lens Reflex or simply SLR.