Outer plasmic shell

The outer plasmic shell of a TARDIS was a component which could be altered to change the outward appearance of the TARDIS by means of its dimensionally transcendental properties. The real world interface on the outer plasmic shell — also known as the door — enabled users to enter or leave the ship. With models such as the Doctor's TARDIS, a Type 40 TARDIS, the outer plasmic shell responded to a chameleon circuit, which configured the shape to best fit in with the TARDIS' surroundings. (TV: Logopolis)

Later, more sentient TARDISes could decide what to change into all by themselves. (PROSE: Alien Bodies)