Definition of brain
(Entry 1 of 2)
1a
: the portion of the vertebrate central nervous system enclosed in the skull and continuous with the spinal cord through the foramen magnum that is composed of neurons and supporting and nutritive structures (such as glia) and that integrates sensory information from inside and outside the body in controlling autonomic function (such as heartbeat and respiration), in coordinating and directing correlated motor responses, and in the process of learning — compare forebrain, hindbrain, midbrain
b
: a nervous center in invertebrates comparable in position and function to the vertebrate brain
(2)
: intellectual endowment : intelligence
—often used in plural plenty of brains in that family
b(1)
: a very intelligent or intellectual person
(2)
: the chief planner within a group
—usually used in plural she's the brains behind their success
3
: something that performs the functions of a brain
especially
: an automatic device (such as a computer) for control or computation
brain
verb
brained; braining; brains
Definition of brain (Entry 2 of 2)
Buying Guide
Check out the best educational products selected by our Reviews team.
Test Your Vocabulary
Eponyms: Words Named After People
-
- What is the best definition of boycott?

Test your knowledge - and maybe learn something along the way.
TAKE THE QUIZ
A daily challenge for crossword fanatics.
TAKE THE QUIZLove words? Need even more definitions?
Merriam-Webster unabridged
Share brain
Time Traveler for brain
The first known use of brain was before the 12th century
See more words from the same century