In the walls of the cubicle there were three orifices. To the right of the speakwrite, a small pneumatic tube for written messages, to the left, a larger one for newspapers; and in the side wall, within easy reach of Winston's arm, a large oblong slit protected by a wire grating. This last was for the disposal of waste paper. Similar slits existed in thousands or tens of thousands throughout the building, not only in every room but at short intervals in every corridor. For some reason they were nicknamed memory holes. When one knew that any document was due for destruction, or even when one saw a scrap of waste paper lying about, it was an automatic action to lift the flap of the nearest memory hole and drop it in, whereupon it would be whirled away on a current of warm air to the enormous furnaces which were hidden somewhere in the recesses of the building.(pp. 34-35)
In the novel, the memory hole is a slot into which government officials deposit politically inconvenient documents and records to be destroyed. Nineteen Eighty-Four's protagonist Winston Smith, who works in the Ministry of Truth, is routinely assigned the task of revising old newspaper articles in order to serve the propaganda interests of the government. For example, if the government had pledged that the chocolate ration would not fall below the current 30 grams per week, but in fact the ration is reduced to 20 grams per week, the historical record (e.g. an article from a back issue of the Times newspaper) is revised to contain an announcement that a reduction to 20 grams might soon prove necessary, or that the ration, then 15 grams, would soon be increased to that number. The original copies of the historical record are deposited into the memory hole. A document placed in the memory hole is supposedly transported to an incinerator from which "not even the ash remains". However not all things tossed in make it to the incinerator.
The term now generally refers to the alteration or outright disappearance of inconvenient or embarrassing documents, photographs, transcripts, or other records, such as from a web site or other archive. The term is the name of one website (The Memory Hole) whose goal is to preserve those documents which are in danger of being lost.