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. He later appeared on-screen as the Doctor in an episode of Doctor Who which was played on a large screen behind Claudia and Dermot. (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)

Other realities
A parallel version of Matt Smith also played the Doctor in a different reality where the Doctor's life was a work of fiction.

He starred in the show's seventh series, alongside Karen Gillan, Arthur Darvill, and Jenna Coleman.

When the Eleventh Doctor accidentally slipped into this reality in 2013, the two met at Doctor Who Con 2013, and Smith mistook him for a an impressive cosplayer. (COMIC: The Girl Who Loved Doctor Who)

Behind the scenes
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". However, this was not an in-universe illustration of Matt Smith, as it was not within the actual narrative of the webcast.

Information from invalid sources
Matt Smith recalled in an interview how nervous he was at the audition, and how everyone told him that he had very "Doctor Who" hair (which, unbeknownst to him, was shaped like a giant question mark). (NOTVALID: The Mind Probe, Doctor Whoah! 417)

He participated in the 2012 Olympics torch relay, where he passed the flame on to the Tenth Doctor. (NOTVALID: Doctor Whoah! 450)