Outer plasmic shell

An outer plasmic shell, often simply called a plasmic shell, was a TARDIS exterior.

Characteristics
In a perfectly functioning TARDIS, the chameleon circuit picked a plasmic shell suitable to the time and place in which the ship landed. A TARDIS' shells were stored in the shell room; if the room had no suitable shells, more were created and stored as well. (PROSE: The Little Things) Lord Roche was forced to use a non-terrestrial aeroplane as his shell when he didn't have any terrestrial designs in his shell room and didn't feel like designing any. (PROSE: The Suns of Caresh) The theory in the TARDIS Handbook was that the plasmic shell was driven by the chameleon circuit. (TV: Logopolis) When a TARDIS died or was dying, its final plasmic shell might grow to enormous size due to leakage of its dimensionally transcendental interior. (TV: The Name of the Doctor)

The Second Doctor boasted that the TARDIS was made from the "hardest materials in existence", making it near impervious to harm. (COMIC: The Monsters from the Past) Indeed, firepower from Dalek gunsticks were unable to penetrate the plasmic shell. (AUDIO: Beneath the Viscoid)

used his chameleon circuit to bond his TARDIS' plasmic shell with the Third Doctor's. (PROSE: The Quantum Archangel)

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

When the Doctor's TARDIS entered siege mode, its exterior took the shape of a cube etched with Gallifreyan writing. (TV: Flatline)

The Doctor's TARDIS
Due to a malfunction in the chameleon circuit, the Doctor's Type 40 TARDIS was stuck with a police box plasmic shell after leaving 1963 London. (TV: An Unearthly Child, COMIC: Hunters of the Burning Stone) Apart from the Sixth Doctor's brief and somewhat limited success in repairing it (TV: Attack of the Cybermen), it retained that shape up through its final resting place as the Doctor's tomb on Trenzalore in an alternate timeline. (TV: The Name of the Doctor)

Among plasmic shells used by the Doctor's TARDIS before the malfunction were: an Ionic column and a sedan chair, (TV: An Unearthly Child) a wooden kiosk, (AUDIO: Quinnis) a boulder, (PROSE: Frayed) a shed (PROSE: The Price of Conviction) and a post box. (PROSE: The Little Things)

The shell of the Doctor's TARDIS had deteriorated by the time Romana I travelled with the Doctor, as after a near miss by a missile in Atrios orbit, (TV: The Armageddon Factor) she designed a routine to strengthen it, though the Doctor disabled it as he abhorred tampering. An action he regretted when a missile was aimed at the TARDIS in Proxima Major orbit. (AUDIO: The Dalek Contract)