Unperson is a person who had been "vaporized"; who has been not only killed by the state, but effectively erased from existence. Such a person would be written out of existing books, photographs, and articles so that no trace of their existence could be found in the historical record. The idea is that such a person would, according to the principles of doublethink, be forgotten completely (for it would be impossible to provide evidence of their existence), even by close friends and family members, and mentioning his/her name is thoughtcrime. (The concept that the person may have existed at one time, and has disappeared, cannot be expressed in Newspeak.) Compare to the Stalinist practice of erasing people from photographs after their death.
excerpt from Wikipedia:
A similar punishment, damnatio memoriae, was used in the Roman Empire. The Soviet Union also provided real-world examples of unpersons in its treatment of Leon Trotsky and other members of the Communist party who became politically inconvenient. In his 1960 magazine article "Pravda means 'Truth'", reprinted in Expanded Universe, Robert A. Heinlein argued that John Paul Jones and a mysterious May 15, 1960 cosmonaut had also received this treatment.