The artifact lands heavily contributed to the dominance of the affinity-based deck archetypes, and thus, in March 2005, all six were banned from Standard (Darksteel Citadel however, was later reprinted in Magic 2015, and the ban did not carry over with the reprint.) In the Modern format, all but Darksteel Citadel were banned from the beginning.
A combination of artifact and land that’s subject to the rules for both. Artifact lands can only be played as lands, not cast as spells. See rule 301, “Artifacts,” and rule 305, “Lands.”