The signature feature of the SLR camera was its mirror, which reflected the view from the lens up into the viewfinder assembly above, and which flipped out of the way when you wanted to take a picture. This worked for many decades, until the birth of the MILC (mirrorless interchangeable lens camera) or plain old mirrorless camera.
These don’t have a mirror, nor an optical viewfinder, instead taking the feed from the digital sensor and sending it to an electronic viewfinder that’s just a small screen. This allows camera manufacturers to make their bodies smaller, as the gap between the back of the lens is narrower because there's no mirror, and the top of the camera can be lower-profile as there's no optical viewfinder.
