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Chakra, from the sanskrit word that translates as "wheel" or "turning", is a concept referring to wheel-like vortices (singular-vortex) which according to traditional Indian medicine are believed to exist in the surface of the etheric double of man.  They are said to be "force centres", rotating vortices of subtle matter and are considered the focal points for the reception and transmission of energies.  Seven major chakras or energy centres are generally believed to exist.

They are typically depicted as either flower-like or wheel-like.  When flower-like, a specific number of petals are shown around the perimeter of a circle, when wheel-like, a certain number of spokes divide the circle into segments that make it resemble a wheel.  Each chakra possesses a specific number of segments or petals.

An alternative explanation: 

A chakra is believed to be a centre of activity that receives, assimilates and expresses life force energy.  The word literally means a wheel or disk and refers to a spinning sphere of bio-energetic activity emanating from the major nerve area forward from the spinal column.  Generally six of these wheels are described, stacked in a column of energy that spans from the base of the spine to the middle of the forehead and there is also a seventh which is beyond the physical region.  It is the six major chakras that correlate with basic states of consciousness.

Further reading: 
Chakras
Chakras