Is there SLR camera motion detect?

Is there a SLR with motion detect. Say you want to set focus on a certain area; and once the subject enters that area of sharp focus, the camera snaps by itself? Is there any model Nikon or Cannon with this fucntion? What is it called? If yes; any clue on the settings? Please…

The technique you want is called trap focus. It can be done with a trick on most higher Nikon models. Not sure about canon.

Trap focusing requires you to be holding the camera - it is not an 'Auto' feature. There are shutter releases that work on sound or the breaking of a light beam, but these are purchased separately & are fairly specialist items (unless you are an electronic geek - circuit diagrams are readily available).