An artifact is a rare and unique object. Most players sacrifice to their god because gods sometimes provide artifacts as gifts. Some NetHack players like to wish for artifacts, although it has been argued that a Barbarian pretending to know the correct spelling of the Eye of the Aethiopica is somewhat out-of-style.
The, the English definite article, is placed before the name of any artifact. For example, the blessed +2 Magicbane or the Orb of Fate. True to grammar, there can only be one of each artifact; if one that has already been generated (or your own quest artifact) is loaded in a bones level, it is converted into its base item type.