Sign language (also referred to as gestural language) is a language where communication is through hand movement, instead of sound. As a language, the use of gestures and hand signals pre-dates the spoken word in most cultures, the major exception being the Leyrons of Malkus IX who actually developed a written language first.
Sign language is permitted on Betazed for the joining ceremony. When Victoria Miller and Lwaxana Troi were debating on who should officiate at the wedding, Troi proposed that her valet Homn would do the officiating. Steven Miller argued that Homn couldn't speak to which Troi responded, "He is quite adept in the act of sign language." (TNG: "Haven")
While on a mission in the Ramatis system, Lt. Commander Data learned and stored five different forms of sign language in order to communicate with Riva, a deaf mediator from Ramatis III. Lt. Worf found the concept of sign language interesting, as its qualities of being both silent and covert could be useful in the field. (TNG: "Loud As A Whisper")