cue dot

A cue mark, also known as a cue dot, a changeover cue or simply a cue is a visual indicator used with motion picture film prints, usually placed on the right-hand upper corner of a frame of the film

Most cue marks appear as either a black circle (if the physical hole is punched out on the negative used to make the projection print of the film), or a white circle (if the mark is made by punching a hole or scraping the emulsion on the film).