"It's bigger on the inside than on the outside!"
- Every companion when they first enter the TARDIS.
Nearly as ubiquitous as the Doctor themself and more so than say the Sonic Screwdriver is the Doctor's TARDIS.
The TARDIS, an acronym for Time And Relative Dimensions in Space, is the Doctor's time and space travel machine. It is powered by a captured black hole known as the Eye of Harmony. It can (or at least is supposed too) blend in with any environment, and most famously it is bigger on the inside than on the outside.
The Doctor's TARDIS (and it is never "Tardis" or "tardis") is presently stuck in the shape of a late 1950s early 60s style Police Public Call Box. The mechanism that allows it to blend in, the Chameleon Circuit, was broken, so when he landed in 1963 it was stuck in that form can could not change to anything else. The Doctor tried to fix it on a couple of occasions, but it seems now (at least implied by Donna in her DoctorDonna incarnation) he just doesn't want to fix it. The Sixth Doctor tried to fix it ("Attack of the Cybermen") but it still ends up not working right.
We learn from the 10th Doctor that a TARDIS was not really built but grown on Gallifrey. And there seems to be an organic structure to them as seen in "The Doctor's Wife" when his TARDIS' consciousness is given a voice.
The Doctor's TARDIS is old. Typically referred to as a "Type 40" or even a "Mark 1" this TARDIS has been called a "museum piece" by River Song on one occasion. When we encounter "The Fugitive Doctor" (Jo Martin) for the first time, her TARDIS looks almost brand new. Leading more credence to the idea that her Doctor was removed from the Doctor's memories a very long time ago.
The TARDIS has the ability to rearrange its own internal environment as seen in more than a few episodes. The ability to add or remove rooms as needed. And even change the configuration of the control room, or as the newer Doctors have put it, "changing the desktop." Something that usually prompts an older Doctor to comment, "Oh, you've redecorated. I don't like it." In fact, it has become something of a running gag in Doctor Who.
As much as "The Enterprise" is a character in Star Trek, the TARDIS is even more so. It has a mind and will of it's own. And as the TARDIS herself (in the form of the wonderful Suranne Jones) says:
The Doctor: You didn't always take me where I wanted to go.
Idris/The TARDIS: No, but I always took you where you needed to go.
Which is true.
To summarize the TARDIS in a single blog post is folly. Sixty years of television has made for a lot to say.
All images are used with permission from the BBC and are copyrighted 2023 by the BBC.