What is an SLR ?

SLR stands for Single Lens Reflex. As the name suggests it has only one (single) lens attached to the body of the camera. An SLR camera can be broadly considered to have two major components. The lens and camera body. The Lens can be attached or removed from the camera body. Since there is only one lens attached to the body of the camera the term Single lens is used. All SLR cameras use a mirror and a pentaprism 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.

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.