Digital single-lens reflex camera

From Wikipedia

A digital single-lens reflex camera (also called a digital SLR or DSLR) is a digital camera combining the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor, as opposed to photographic film. The reflex design scheme is the primary difference between a DSLR and other digital cameras. In the reflex design, light travels through the lens, then to a mirror that alternates to send the image to either the viewfinder or the image sensor. The alternative would be to have a viewfinder with its own lens, hence the term "single lens" for this design. By using only one lens, the viewfinder presents an image that will not perceptibly differ from what is captured by the camera's sensor.

DSLR's have largely replaced film-based SLR (Single-lens reflex camera's) since 2000's and are the most preferred type of camera for photographers.

The photographer can see the subject before taking an image by the mirror. When taking an image the mirror will swing up and light will go to the sensor instead.