Archive footage is the term given to footage from stories that ends up being reused in later stories. It's used as a flashback or to make a character seem there. The process is more common in the revival of Doctor Who but this has occurred in the "classic" stories Logopolis and the beginning of The Five Doctors.
The process has also been used not as a flashback but to incorporate characters into a story. The Name of the Doctor and The Day of the Doctor take archival footage out of their original contexts, and uses them so it seems the Doctors are there in the story.