Fourth wall
The fourth wall is a theatre term referring to the audience. This originates in the idea that there are three walls on a stage: one on the back, one to the left, and one to the right, as well as an imaginary fourth wall in front that contains the players within their play. To "break the fourth wall" means to show awareness of the audience or other things outside of it.
The fourth wall was famously broken in episode 7 of The Daleks' Master Plan, The Feast of Steven, in which the First Doctor wishes the viewers a merry Christmas: "Oh and incidentally, a happy Christmas to all of you at home."
At the end of The Invasion of Time, the Fourth Doctor breaks the fourth wall by grinning mischievously to the camera.
At the end of The Trial of a Time Lord's last story, The Ultimate Foe, the Valeyard breaks the fourth wall by looking directly into the camera and laughing.
In the proms special, Music of the Spheres, the Tenth Doctor breaks the fourth wall by addressing the audience at the Royal Albert Hall.
At the end of the Series 1 finale of K9, The Eclipse of the Korven, K9 breaks the fourth wall by looking directly at the camera and saying "affirmative".
