The November 1988 worm perpetrated by Robert T. Morris. The worm was a program which took advantage of bugs in the Sun Unix sendmail program, Vax programs, and other security loopholes to distribute itself to over 6000 computers on the Internet. The worm itself had a bug which made it create many copies of itself on machines it infected, which quickly used up all available processor time on those systems.
Some call it "The Great Worm" in a play on Tolkien (compare elvish, elder days). In the fantasy history of his Middle Earth books, there were dragons powerful enough to lay waste to entire regions; two of these (Scatha and Glaurung) were known as "the Great Worms". This usage expresses the connotation that the RTM hack was a sort of devastating watershed event in hackish history; certainly it did more to make non-hackers nervous about the Internet than anything before or since.