The term 'silent signal' refers to a communication method that does not involve speaking. It is often used when verbal communication is not possible or appropriate, such as in a library or a theater. Two common silent signals are the NOD (3 letters) and GESTURE (7 letters). The NOD is a quick up-and-down movement of the head, usually to indicate agreement or understanding. A GESTURE is a movement of the body, arms, or hands to express an idea or emotion.