In dentistry, MOD stands for mesial-occlusal-distal. MO (mesial-occlusal) is used to describe the surfaces of a tooth that need treatment or are filled. Mesial is the surface of tooth nearest the middle of your mouth; Occlusal is the top or biting surface of a back tooth; Distal are the front or back part of the tooth, respectively. This is just one form of the many shorthands used by dentists in dental examination. Dentists use alpha-numeric jargons like 3 MOD, 5 DO, 13 MFD and so on. The numbers represent specific teeth, and the letters describe areas of those teeth.