Final answer:
Trismus is defined as the inability to fully open the mouth due to a prolonged contraction or spasm of the jaw muscles, often seen after a closed head injury or in association with conditions like tetanus.
Step-by-step explanation:
When assessing a patient with a closed head injury, trismus is defined as a condition where there is a prolonged contraction or spasm of the jaw muscles, which results in the inability to open the mouth fully. This can be a reflex response to the injury or associated with various medical conditions, such as tetanus, which is known for causing a rigid body posture called opisthotonos. In the context of head injury, trismus may indicate involvement of the muscles or nerves controlling jaw movement and should be promptly evaluated as part of the patient's condition.