脑海 (nǎo hǎi) is a figurative noun meaning 'the mind' as a mental space where thoughts, memories, and images reside — like a 'sea of the brain.' It differs from 大脑 (dà nǎo, the physical brain organ), 脑子 (nǎo zi, brain/intelligence), and 脑袋 (nǎo dai, colloquial head/brains), which all refer to the literal organ or intellectual capacity. 脑海 is used only in metaphorical expressions like 在脑海里 (in one's mind).