Fade to black

Two seconds of black in the middle of a video works as a full stop, and viewers read it that way without ever being taught. The transition takes the image down to a flat colour over anything from a few frames to a couple of seconds, and usually fades back up on whatever comes next. Length carries the meaning, so a quick dip suggests a small step forward in time and a long hold suggests the end of something. Other colours work too, with white common for a clean or clinical feel. Used mid-video it carries an obvious risk, since anything resembling an ending gives a viewer permission to leave, which is why plenty of editors save the full fade for the last frames.