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

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

Any mass educational system of improving or providing conditions to increase the intellectual quality and yield of a population of (human) brains, e.g. a university or school.

A condition to which male high school and college students are prone consisting of difficulty in concentrating, mild forgetfulness, and difficulty in maintaining a train of thought, arising from "too much thinking". Physical symptoms can manifest as well.

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

Someone who does brain surgery. / (idiomatic) Someone very intelligent.

Synonym of knowledge worker

(chiefly US) Originally, a group of experts who advise a government, often informally. / More recently, any group of experts assembled to give advice.

impetuous and rash; hot-headed

(medicine) An irreversible loss of brain function and cessation of brain activity.