A magic word implemented by Will Crowther in the original text adventure, known as "Advent" or "Colossal Cave Adventure", among other names. It was the first magic word the player encountered, and transported the PC between two locations on incantation. It is now included in many current games as an homage.
The Zork games from Infocom had a response to xyzzy (as well as the additional spells plugh and plover): "A hollow voice says, 'Fool.'" (In Advent, a hollow voice is what informs the player of the magic word "plugh".) Many modern IF games have a humorous response to the magic words (or just >XYZZY) as well, sometimes involving the hollow voice.