Esoteric teaching on this word varies greatly from source to source.
All sources agree that this is a sort of force or effect that is localized at the base of the spine and can be activated either spontaneously or through deliberate exercises. Kundalini is said to rise from the first chakra, along the spine, possibly all the way to the crown chakra. It is sometimes compared to a serpent that lies coiled at the base of the spine. Awakening this serpent is claimed to bring great powers and benefits.
Gurdjieff's view on the matter differs from most sources. He agrees that such a thing exists but teaches that it is the source of false imaginings, an actual bane of man, the remnant of the ill-famed 'organ kundabuffer' which is discussed extensively in Beelzebub's Tales to his Grandson. Kundalini is in effect the spell by which the 'evil magician' has hypnotized man to believe himself to be a magician while in truth he remains a sheep. Awakening the kundalini causes man to go live deeper in illusion and is ruinous to the Work, says Gurdjieff.
The kundalini is often compared to a serpent, maybe because of the undulating sensation that can be felt along the spine when doing certain exercises. Mouravieff writes that this is what the serpent of Genesis actually represents.
See Evil Magician.