The Big Deletion was a galaxy-wide computer virus event that wiped out most of humanity's electronically stored information about the past, and also led to the destruction of many extrasolar human colonies.
Behind the Scenes
The concept of The Big Deletion was created to explain the inconsistencies between the Aliens and Aliens vs. Predator comic books published by Dark Horse Comics, and the film Alien Resurrection — specifically, it concerns a line of dialogue from General Peréz in the film, in which he states states that the Xenomorph was essentially wiped out following Ellen Ripley's sacrifice on Fiorina "Fury" 161 (as seen in Alien3), and that the species has been extinct to mankind's knowledge for the last 200 years.
This scenario contradicts the comics, in which humans came into repeated contact with the Xenomorphs in the interim period. In fact, a large amount of often fruitful research into the creatures and their biology would be carried out by different human scientists during this time. At one point, the Earth itself even became infested by the creatures on a global scale and was ravaged as a result (as portrayed in Aliens: Outbreak, Aliens: Nightmare Asylum and Aliens: Female War). This monumental event would take humanity close to a generation to recover from.
Thus, The Big Deletion was devised as a means by which humans in the future period of Alien Resurrection might have no knowledge of these prior events. The deletion is first mentioned as a background event that occurred some time previously in the comic Alien vs. Predator: Thrill of the Hunt, while the nature of The Big Deletion and its ramifications on human society are further explained in the sequel Alien vs. Predator: Civilized Beasts. Therein it is revealed that human-imitation synthetics were the primary vector of the virus' spread, and that humanity subsequently outlawed and marginalized such synthetics as a result — which also tied in to Alien Resurrection, in which it is revealed that synthetics and autons have long since been declared illegal and were subject to a forced dismantling program.