脖 is a bound morpheme (b.f.) meaning 'neck'. It cannot be used alone as a full word—you must attach the suffix 子 (forming 脖子 bózi) or combine it with another morpheme (e.g., 脖颈 'back of the neck', 脖套 'neck warmer'). In everyday speech, 脖子 is the standard word for 'neck'.