(informal) The temporary inability to think properly, or to remember something

(pathology, dated) encephalitis or meningitis

(pathology) Alternative spelling of brain fever

A complex riddle or puzzle.

(slang, derogatory) A fool.

(derogatory) A foolish or dull-witted person.

Surgery on any part of the nervous system, especially the brain; neurosurgery. / (uncountable, idiomatic) Something that is overly complex, detailed or confusing.

Any of several species of coral with the appearance of a brain, in the families Mussidae and Merulinidae.

(cytology) A cell in the brain, such as a neuron or a glial cell. / (colloquial) neuron / (humorous) -- taken as a unit of intelligence

(figuratively, slang, mildly vulgar) Something ill-considered and said or done impulsively. / (figuratively, slang, mildly vulgar) An instance of absent-mindedness or forgetfulness.