a. The portion of the vertebrate central nervous system that is enclosed within the cranium, continuous with the spinal cord, and composed of gray matter and white matter. It is the primary center for the regulation and control of bodily activities, receiving and interpreting sensory impulses, and transmitting information to the muscles and body organs. It is also the seat of consciousness, thought, memory, and emotion.
b. A functionally similar portion of the invertebrate nervous system.
2. The mind: The plan took shape in her brain.
a. Intellectual ability; intellect: an actor not known for his quick brain.
b. often brains Exceptional intellectual ability; intelligence: has brains and good looks.
c. Informal A highly intelligent person: We knew the new kid was a brain as soon as she started talking.
4. often brains The primary director or planner, as of an organization or movement.
5. The control center, as of a ship, aircraft, or spacecraft.
tr.v. brained, brain·ing, brainsIdioms:
To hit on the head or kill by hitting on the head.
beat (one's) brains (out)
Informal To exert or expend great mental effort: She beat her brains out during the examination.
on the brain
Obsessively in mind: The coach has winning on the brain.
pick (someone's) brain(or brains)
To explore another's ideas through questioning.
rack (one's) brainInformal
To think long and hard: I racked my brain for hours trying to recall her name.
[Middle English, from Old English brægen.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.