1. Motor disturbance of the trigeminal nerve, especially spasm of the masticatory muscles, with difficulty in opening the mouth, a characteristic early symptom of tetanus.
2. A pathological, sustained spasm of the neck and masseter (jaw) muscles that can make it difficult or impossible to open the mouth.

It is most often associated with tetanus.

3. A tonic contraction of the muscles of mastication.

More recently, the term trismus has been used to describe any restriction to the opening of the mouth, including restrictions caused by trauma, surgery, or radiation.

4. A spasm of the jaw muscles which makes it difficult to open the mouth.

Congenital adhesion of the cheeks to the gums.

Stiffness of the jaw muscles in neonates; usually, as the beginning of tetanus neonatorum.

Tetanus occurring in newborn infants, usually due to infection of the umbilical area with Clostridium tetani, often a result of ritualistic practices; has high fatality rate (about 60%).