Trailer

A trailer (also known as a preview) is a commercial advertisement, originally for a feature film that was going to be exhibited in the future at a movie theater/cinema, but has also been attributed to television shows as well as video games and other sorts of content material for the brand. It was a product of creative and technical work.
Trailers have been a part of the Doctor Who franchise from the very beginning in 1963 as small snippets of the show, and has continued to be part of the show to this day. Trailers varied from anything between 30 seconds and 2 minutes.
In the early days, the BBC would air a single simple trailer just to give away the basics of what the audience was going to get in from a story. In the 21st century trailers have been anything from the action of the story to specially filmed content just for the trailer, even entirely new narratives. The amount of trailers were also changed. Trailers could differ between being set in the "DWU" and being out-of-universe.