The world tree is the motif of a great tree connecting the levels of the cosmos: roots in the underworld, trunk in the world of the living, branches in the heavens. It is the most common single form of the axis mundi.
Yggdrasil in Norse cosmology is the fullest developed example, an ash whose three roots reach to three wells, gnawed at from beneath by the dragon Nidhogg while a squirrel carries insults up and down between the dragon and an eagle at the top, with the Norns tending it at the well of Urd and the gods holding their assembly beneath it. Odin hangs on it nine nights, wounded with a spear, "myself given to myself", to win the runes. It shudders at Ragnarok but survives, and two human beings shelter within it while the world burns.
Siberian and Central Asian traditions have the tree as the shaman's route between worlds, and the ritual pole or notched trunk climbed during a seance is understood as its representation, which is one of the clearest cases of a cosmological image built into practice. Mesoamerican traditions place a world tree at the centre with four more at the quarters, the ceiba being its usual identification among the Maya. Indian tradition has the ashvattha of Bhagavad Gita 15, described with its roots above and its branches below, inverted, since its source is not in this world, and the text instructs that it be cut down with the axe of detachment. The Bible has the tree of life in Eden and again at the close of Revelation, with the tree of the knowledge of good and evil standing beside it in the first instance. Buddhist tradition has the Bodhi tree, which is a specific historical tree at a specific place rather than a cosmic one, though the tree beneath which a Buddha attains awakening is described in cosmological terms and is held to stand at the centre of the world.
The motif's practical expressions include sacred groves and individual sacred trees, which are among the most durable of religious sites and among the most frequently destroyed in campaigns of conversion, the Irminsul of the continental Saxons, felled by Charlemagne in 772, and the oak at Geismar felled by Boniface. It is also invoked for the maypole and the Christmas tree, whose pre-Christian ancestry is often asserted and is in fact poorly evidenced; the Christmas tree is documented from early modern Germany, and its supposed pagan origin is largely a nineteenth-century construction.
That last point illustrates a general caution. The world tree is a genuine and widely attested comparative category, and it is also among the categories most freely used in popular writing to assert continuities the evidence does not support.