Roundel

A roundel (TV: Terminus) was a key feature of the Doctor's TARDIS. Roundels were mainly used as a decorative feature, although occasionally they did actually fulfil a function.

The First Doctor's TARDIS had basic white roundels on the wall, circular in shape. They were indented into the wallpaper. (TV: An Unearthly Child)

The Third Doctor added plastic furnishings to the edges of the roundels. One of them then became the replacement for the scanner. (TV: Day of the Daleks, The Curse of Peladon, The Time Monster) Later, his TARDIS reverted to its original design. (TV: The Three Doctors)

Some of these roundels doubled as access covers to circuitry in the walls, at least during the Fifth Doctor's tenure. (TV: Terminus)

The Eighth Doctor's TARDIS had circular cut-outs in the support beams. (TV: Doctor Who)

In the Ninth Doctor's first control room, the walls of the TARDIS featured backlit hexagons. (TV: Rose) By the time of the Tenth Doctor, overhead lights were added to the TARDIS' illumination, so the roundels fulfilled a mainly decorative function. (TV: The Runaway Bride) The first of the Eleventh Doctor's TARDISes had not so many roundels in the walls, but the theme did continue slightly, and there was a larger circular screen set into the wall near the door. One of his console rooms had blue and red neon circles. (TV: The Snowmen)

When the Tenth Doctor's TARDIS interior glitched due to the presence of the War Doctor and the Eleventh Doctor and reverted to the War Doctor's desktop settings, the Eleventh Doctor pointed out the "round things" to his predecessor. The Tenth Doctor voiced his love for the "round things", though he admitted he had no idea what they were when his successor asked. (TV: The Day of the Doctor)

Behind the scenes
Though a common enough word in fandom, roundel is a tough word to find in a televised script. However, it has happened, perhaps most clearly in Terminus, where Turlough, Tegan and the Black Guardian use the word several times at the top of the first episode.