Eldritch is an adjective of archaic origin denoting something strange, eerie, and alien in a manner that transcends ordinary fear — suggestive of forces wholly outside human understanding. Lovecraft employed it with near-signature frequency to compress an entire atmosphere of cosmic alienation into a single modifier.
Within the genre, "eldritch" now functions as an aesthetic category in its own right, distinguished from mere terror by its implication of ontological wrongness.