A disappearance was when someone or something vanished.
History
In 1930, reports of disappearances from the New York City's Hoovervilles flooded in, the result of the Cult of Skaro abducting residents to convert them into either Dalek-Humans or pig slaves. (TV: Daleks in Manhattan [+]Helen Raynor, Doctor Who series 3 (BBC One, 2007).)
Susan Foreman, the Doctor's granddaughter (TV: "An Unearthly Child"), was considered to have disappeared (PROSE: Rose sighting confirmed [+]BBC webteam, Who is Doctor Who? (BBC, 2005).) in 1963 after she departed London in the TARDIS with the First Doctor and teachers Barbara Wright and Ian Chesterton, who were abducted by the Doctor so they couldn't reveal the existence of the TARDIS. (TV: "An Unearthly Child") Susan's disappearance led her to eventually be placed on Coal Hill School's Roll of Honours Board. (TV: For Tonight We Might Die [+]Patrick Ness, Class television stories series 1 (BBC Three, 2016).) In 2005, rewboss, a reader of whoisdoctorwho.co.uk, a conspiracy website which occasionally tracked the Doctor's appearances on Earth, ridiculed a suggestion that the Doctor was a time traveller before suggesting the webmaster wanted to tie him in with Susan's disappearance. (PROSE: Rose sighting confirmed [+]BBC webteam, Who is Doctor Who? (BBC, 2005).)
Other companions of the Doctor would also be considered to have disappeared; Rose Tyler left 2005 with the Ninth Doctor (TV: Rose [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005)., PROSE: Rose [+]Russell T Davies, adapted from Rose (Russell T Davies), Target novelisations (Target Books, 2018).) and was accidentally returned home one year later, during which time a missing persons case had been opened and her boyfriend Mickey Smith was suspected of her murder. (TV: Aliens of London [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005).)
References
The UNIT website's disclaimer page included "disappearance" on a list of bodily ailments which could potentially occur while reading the website which the disclaimer legally absolved UNIT from being responsible for. (PROSE: Privacy Policy, Disclaimer and Terms of Use [+]BBC webteam, U.N.I.T. (BBC, 2005).)