内部 (naibu) means “inside” | MotiTan Japanese Dictionary

More examples

箱の内部には何もありません。bako no nai bu ni wa nan mo a ri ma se n

There is nothing inside the box.

Identifies the space or part contained within the boundaries of something.

中naka: The space inside something, often used in casual everyday speech.
内側uchigawa: The part or surface on the inside of something.

物や建物などの内側にある部分。mono ya tate mono na do no uchi gawa ni a ru bu bun

The noun 内 (nai, "inside") combines with 部 (bu, "part") to form 内部 (naibu), meaning the inner part of something.

In everyday Japanese, 内部 (naibu) sounds more formal and precise than casual wording, so it commonly appears in explanations about a car, building, or device.