A sign of the M-9 language
M-9 was the designation of a sign language using two hands.
In 2365, this language had been recorded in the linguistic database of the United Federation of Planets. It was identified and learned by Lieutenant Commander Data in his attempt to communicate with Riva, a deaf mediator from Ramatis III. M-9 was one of five languages he chose to learn to identify the particular language Riva was using. As such, Data viewed multiple M-9 gestures on a screen.
One of the gestures of the M-9 sign language was similar to the Vulcan salute. (TNG: "Loud As A Whisper")
According to the reference book Star Trek Encyclopedia (4th ed., vol. 2, p. 7), M-9 was originated by the Leyrons. The book also mentions that most of the gestures seen on Data's screen were adapted from American Sign Language (which makes sense, as Riva actor Howie Seago used ASL), and the Vulcan salute was included "to see if anyone would notice."