Newspeak for holes in Party offices for the disposal of documents and other records. It leads down a shaft to a furnace. Winston Smith edits historical records, news articles, and other documents to reflect changes in the current situation, and dumps any incriminating or "outdated" records down the memory hole. Like most doublespeak words, it has contradictory, euphemistic shades of meaning. The memory hole serves to destroy the "ungood" memory of the true events and establish falsehood as the absolute truth.
Hence, when Oceania is at war with Eurasia, it has always been at war with Eastasia. The records show it. When the party executes a man for thoughtcrime, he ceases to have existed; there is no trace of him in records anywhere.