The Missing Man formation was an aerial salute first used by the flying forces of Earth's militaries in the 20th century, as a way to honor a fallen comrade. The formation normally consisted of a delta-shaped pattern of aircrafts, from which a single craft quickly pulls away, leaving an empty space in the formation.
The tradition continued into the era of spaceflight. Shortly after the death of James T. Kirk aboard the USS Enterprise-B in 2293, Ambassador Spock, Dr. Leonard McCoy, and Commander Nyota Uhura unconsciously replicated a Missing Man formation while beaming down to Mestiko for a planetary summit. (TOS - Mere Anarchy eBook: Its Hour Come Round)
In the alternate reality created by Nero's temporal incursion. the missing man formation was also used in the year 2259 to honor those who had perished in the attack on Starfleet headquarters in San Francisco. (ST movie: Star Trek Into Darkness)