The word "脑海" (nǎohǎi) is formed through a vivid conceptual metaphor where the mind is imagined as a sea. The first character, "脑" (nǎo), literally means "brain", representing the physical organ and the seat of thought. The second character, "海" (hǎi), means "sea" or "ocean". They combine to create the poetic image of the mind as a vast, deep, and fluid expanse where thoughts, memories, and ideas exist like waves or contents within an endless sea, thus conveying the abstract concept of the "mind's eye" or the expanse of one's thoughts and consciousness.