The axis mundi (also cosmic axis, world axis, world pillar, center of the world, world tree), in religion or mythology, is the world center or the connection between Heaven and Earth. As the celestial pole and geographic pole, it expresses a point of connection between sky and earth where the four compass directions meet. At this point travel and correspondence is made between higher and lower realms. Communication from lower realms may ascend to higher ones and blessings from higher realms may descend to lower ones and be disseminated to all. The spot functions as the omphalos (navel), the world's point of beginning.
The image is mostly viewed as feminine, as it relates to the center of the earth (perhaps like an umbilical providing nourishment)[citation needed]. It may have the form of a natural object (a mountain, a tree, a vine, a stalk, a column of smoke or fire) or a product of human manufacture (a staff, a tower, a ladder, a staircase, a maypole, a cross, a steeple, a rope, a totem pole, a pillar, a spire). Its proximity to heaven may carry implications that are chiefly religious (pagoda, temple mount, minaret, church) or secular (obelisk, lighthouse, rocket, skyscraper). The image appears in religious and secular contexts. The axis mundi symbol may be found in cultures utilizing shamanic practices or animist belief systems, in major world religions, and in technologically advanced "urban centers". In Mircea Eliade's opinion, "Every Microcosm, every inhabited region, has a Centre; that is to say, a place that is sacred above all." The axis mundi is often associated with mandalas.
(Wikipedia)

Yggdrasil (World Ash) - Plants often serve as images of the axis mundi.
Image source: en.wikipedia.org Yggdrasil
Reference: en.wikipedia.org Axis mundi

#2: Mount Kailash, Tibet (Mount Meru in mythology)
Image source: www.flickr.com
References: Wikipedia Mount Kailash 
Mount Meru (mythology)

#5: "The Garden of Eden" by Lucas Cranach der Ältere, a 16th century German depiction of Eden.
Image source: en.wikipedia.org
Reference: Wikipedia Garden of Eden

#6: Yggdrasil,The World Ash tree
Image source: photobucket.com
References: Wikipedia Yggdrasil
World tree

#8: Bodhi tree, Bodh Gaya, India
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References: Wikipedia Bodhi tree 
Bodh Gaya