牙 is the colloquial word for tooth. The more formal/medical term is 牙齿 (yáchǐ). In compounds: 牙医 (yáyī, dentist), 牙膏 (yágāo, toothpaste), 牙刷 (yáshuā, toothbrush).
culture
In Chinese tradition, when a child loses a lower tooth, it's thrown onto the roof; an upper tooth is thrown under the bed. This is believed to help the new tooth grow in the right direction.
