Matt Smith (in-universe)

Matt Smith was an actor in the Doctor Who series. He portrayed the Doctor, or "Doctor Who".

Biography
When the Eleventh Doctor materialised his TARDIS in the studio where Red Nose Day 2011 was being filmed, the screen behind him showed an image of Matt Smith next to a TARDIS and the words "Doctor Who". (TV: The Doctor Drops In)

In Red Nose Day 2013, Claudia Winkleman rhetorically asked how amazing Matt Smith was in Doctor Who. (TV: The Doctor Appears)

He was considered by the Twelfth Doctor to be one of the greatest actors on Earth, and was brought together, along with many other actors to portray "Doctor Who", for a special breakfast in "London's glittering West End". The Twelfth Doctor considered making an appearance, to "see what these fictions [were] like." (WC: Peter Capaldi and Simon the Shy Cyberman Invite You to Breakfast with 7 Doctors)

Behind the scenes
Evidently, "Matt Smith" was intended in all cases above to be an in-universe version of the real Matt Smith, who portrayed the Eleventh Doctor. At the end of Peter Capaldi and Simon the Shy Cyberman Invite You to Breakfast with 7 Doctors, a screen appeared featuring photographs of each Doctor who would be present at the event, in-costume as "the Doctor". This may be taken as an in-universe illustration of Matt Smith, although not within the actual narrative of the webcast.