The axis mundi, world axis, is a comparative category for the point or line at which the levels of the cosmos are joined, where heaven, earth and underworld meet and where passage between them becomes possible. The term belongs to the study of religion rather than to any tradition's own vocabulary, and it was made central by Mircea Eliade, who treated it as one of the fundamental structures of religious space.
Its instances take several forms. A tree: Yggdrasil in Norse cosmology, its roots and branches spanning the worlds. A mountain: Meru in Indian cosmology, Kailash, Zion, Olympus. A pillar or post: the Irminsul of the continental Saxons, the central pole of a Siberian tent or a Plains lodge, which is treated as the route of the shaman's ascent. A ladder or rope: Jacob's ladder at Bethel, and the rope that in many African and Asian myths once joined heaven and earth and was cut, after which the two have been separate. A building: the ziggurat, whose Mesopotamian names describe it as the link between heaven and earth; the stupa; the temple; the spire; and the omphalos stone at Delphi marking the navel of the world.
The category does genuine explanatory work. It accounts for why sanctuaries are so often described as standing at the centre of the world even when there are several of them at once, the centre being a quality rather than a coordinate. It accounts for the recurrence of vertical imagery in accounts of religious ascent and descent. And it accounts for the ritual weight given to thresholds, ladders, poles and doorways.
It has also been criticised, and the criticisms are now standard. Eliade's method assembled instances from widely separated cultures with little attention to their contexts; it imposed a single pattern on materials that meant different things locally; and it treated the pattern as evidence of a universal structure of religious consciousness rather than as a family of resemblances that might have several explanations. Later scholarship has generally kept the descriptive usefulness of the category and dropped the claim about the human mind. Used carefully it names something genuinely recurrent. Used loosely it flattens the traditions it is applied to, which is the risk with every comparative category of this kind.