"Vanishment" was the term for the mysterious disappearance of a spacecraft. It was a term apparently in wide use and, according to K9, something that had been happening "since space travel had first begun".
While trying to solve the mystery of the vanishment of the Rigelian Seventh Fleet, K9 discovered that the Time Lord known as Omegon had been responsible for the disappearance of not only the Rigelian ships, but countless others. He intended to launch his captured ships against Gallifrey in an attempt to gain revenge against his ungrateful fellows. (PROSE: K9 and the Time Trap)