An artifact is any inventory actor which is not automatically used when picked up and therefore remains in the player's inventory, where it can be used when wanted. Heretic, Hexen and Strife all feature a selection of artifacts, whereas Doom and the other games do not; though mods can use artifacts in any game.
The artifact's Inventory.Icon must be defined as it is what will be used to identify it in the inventory bar of the HUD. In the default Doom status bar, if artifacts are available, the mugshot is overridden by a display of the currently selected artifact. The "item" functions ("next item", "previous item", "activate item", "drop item", etc.) all affect the currently selected artifact.
The give artifacts console command will give all defined artifacts to the player.