Shakti means power or energy. Shakti is generally used in Yoga to refer to primordial cosmic energy which also be called [I]adishakti[/I](literally primordial energy) however Shakti can also be used to refer to various forms of energy such as Ichaa shakti(desire energy/will power) and jnana shakti(knowledge energy/knowledge power) The [wiki]prana[/wiki] or life force in ones body is also a form of shakti. The highest form of shakti in the body is the [wiki]kundalini[/wiki] energy said to reside at the base of the spine.
The concept of shakti first appears in the Rig Veda(see [wiki]Vedas[/wiki]) as Aditi. Aditi is the divine mother and the primordial matter or source of creation. The word Aditi being a compound of A+diti, where A means not and diti means bound or finite. Thus, Aditi is the infinite and boundless. From Aditi issue the Adityas, literally meaning those born of Aditi, they are the original Vedic gods. The main ones being: Indra, Surya, Mitra, Varuna. Each of these Vedic gods is associated with a different power or energy. Such as Indra is the energy behind creation; Surya behind sustaining and life giving.
Shakti is also worshipped in the matriarchal Shaktism sect of Hinduism. There Shakti is worshipped in one of her nine forms deified as goddesses. Such as Kali, Durga, Saraswati, Ila, Amba etc. A popular festival that is celeberated by Hindus called Navaratri or 9 nights is dedicated to worshipping the nine forms of Shakti over 9 nights.