Brain

A brain was the central area for thought in most sentient biological entities.

Richard Harries experimented with brainwaves. He had sibling rats transfer knowledge through the waves and complete tasks the other had learnt. (EDA: The Banquo Legacy)

The Fifth Doctor created an anti-dreaming device that stopped the brainwaves associated with dreaming. (DW: Snakedance)

While most brains were natural, the Rani once constructed a giant brain to absorb the mental energies of various individuals and compute the equations for a time manipulator. (DW: Time and the Rani)

In certain circumstances, the brain could be transplanted out of the biological entity into another vessel. This was done with Morbius' brain after his body was destroyed. His ally, Mehendri Solon, had to find a head large enough to fit the brain and only found the right match when he met the Fourth Doctor. (DW: The Brain of Morbius) On Pete's World, the Cybermen there were created by having the brain transplanted from the human form into the cybernetic suit. (DW: Rise of the Cybermen)

The Ood had two brains. One was carried in their hands. There was also a giant brain on the Ood Sphere. (DW: Planet of the Ood)

The Morpho were a race of disembodied brains. (DW: The Keys of Marinus)

K9 Mark 2 told Starkey that the second planet in the Montoon system had an "ocean of brains". (K9TV: Curse of Anubis)

Miranda Cleaves had a blood clot in her brain, as did her ganger. Cleaves was cured by a poultice the Eleventh Doctor gave her. (DW: The Almost People)

The Judoon had rather small brains; a Judoon brain was able to fit in a box that could easily be held in a humanoid palm. (WC: Prequel (A Good Man Goes to War))