brain

When used to describe the physical organ, 'brain' is a countable noun. For example, you can talk about 'the human brain'. When referring to intelligence or mental ability in general, it is often used in singular form (e.g., 'a brain for business'). In casual conversation, calling someone 'the brain' of a group means they are the smartest person or the one who makes all the plans. As a verb, you will almost always see 'brain' as part of the compound word 'brainwash'. It is rarely used on its own to mean indoctrination.