(figuratively) To struggle to think of or remember something.

(medicine) A traumatic injury to the brain caused by an external force.

(dated) attention deficit hyperactivity disorder

(surgery) A neurosurgical procedure involving the implantation of a medical device called a neurostimulator which sends electrical impulses, through implanted electrodes, to specific parts of the brain for the treatment of movement and affective disorders

(anatomy) A structure in the central nervous system (CNS) that keeps various substances found in the bloodstream out of the brain while allowing in the substances essential to metabolic function, e.g. oxygen.

incoherent, idle thoughts "at the top of the brain"

Eggcorn of rack one's brain.

(idiomatic) To struggle to think or remember something.

The return of people who emigrated from a country during the process of brain drain.

(UK, dated, figuratively) to think hard and try to remember something.