A post-credits scene was a scene from an episode of Doctor Who or any of its spin-offs that was present, as the name suggest, at the end of the end credits. It would usually show up directly after the entire crew list had scrolled through on the screen, right before the black screen with the BBC Cymru Wales logo.
A post-credit scene appeared at the end of the series 9 episode Face the Raven. It featured Rigsy finishing painting a mural on the TARDIS's exterior of Clara Oswald in honour of her after she died sacrificing herself to the Quantum Shade on Trap Street to save him.
Doctor Who: Redacted features a post-credit scene at the end of episode 10 Salvation. It featured Cleo Proctor coming home to her brother Jordan Proctor in their shared apartment, a package waiting for her. Upon reading the note, she finds out Madame Vastra sent it, and when she opens it, finds out that it is the Floater, the alien being from another realm, in case she should get into trouble again. Jordan reacts by falling backover as she says they’ve got a new house mate.