In common fan parlance, a multi-Doctor story is one that involves more than one incarnation of the Doctor. Typically, although not necessarily, such tales are reserved for anniversary and/or charity events. The term is not usually applied to stories like The Almost People which involves a doppelgänger, or Father's Day, which has multiple instances of a single incarnation.
They are exceedingly rare on television and audio, but much more common in short stories and comics.
Multi-Doctor stories include: