Single Lens Reflex. A type of camera that has one (switchable) lens that a) projects the image to the film and b) projects the image to the viewfinder.

The finder part is done with a mirror and prism: Upon press-motion of the Button That Causes The Kodak Moment To Be Captured Forever and Ever, the mirror that reflects the image to the prism (and thus to the viewfinder) moves away, permitting the image to be projected through the shutter to the film.

SLR is kind of cool because the picture that's visible from viewfinder is precisely - lens focus, iris, all that jazz - the same that will go to the film (most cheap cameras have separate finder, you know, makes getting perfect pictures in all environments much harder!). My sister noted the noise it makes, though - she said I can always be found easily if I take photos, because of the loud *CLANK!* every time I take one. (My camera is from late 1960s, but I know the modern SLRs aren't much quieter. =)