Thought is an arrangement of ideas created by the brain which allows people to think. There is no real understanding of how it works or how to define it.
Ministry of Magic
In the Department of Mysteries, the Ministry of Magic study thought by keeping brains in a water tank where they swim. As part of their studies, they could use magic to retrieve thoughts, which created tentacles of memories which looked like ribbons of moving images. When the tentacles touched someone, they were capable of hurting them by wrapping their tentacles around them.
In the Battle of the Department of Mysteries, Dumbledore's Army and the Death Eaters fought in the Brain Room. When Ron Weasley was Confunded, he thought the brains looked funny and summoned them. When the thoughts soared at him, they started to wrap themselves around Ron's arms, nearly suffocating him. When the battle ended, Madam Pomfrey said that it would leave a scar, since nothing leaves deeper scarring than thought.
- ""Hey, Harry, there are brains in here, ha, ha, ha, isn't that weird, Harry?...Honest, Harry, they're are brains — look — Accio Brains!" The scene seemed momentarily frozen. Harry, Ginny, and Neville and each of the Death Eaters turned in spite of themselves to watch the top of the tank as a brain burst from the green liquid like a leaping fish...The moment they came in contact with his skin, the tentacles began wrapping themselves around Ron's arms like ropes..."Harry, it'll suffocate him!" screamed Ginny."
- —Ron's attack by thought[src]
- Harry Potter and the Order of the Phoenix (First appearance)
- Harry Potter and the Half-Blood Prince (Mentioned only)