Shakti is a Sanskit word meaning energy or power.
In The Tantrik Goddess, Shri Gurudev Mahendranath wrote:
“The word Shakti means power. It is, according to the Tantriks the Power by which the Absolute manifests itself. The worship of Shakti therefore implies the worship of God as the Mother. In symbolism, the universe is seen to issue from a womb, as in human birth and this implies Motherhood. One who worships Shakti is called a Shakta.”
Shakti or Shakti Devi is also a reference to the consort of Shiva. As Devi, she is the personification of God as the Divine Mother and represents the active, dynamic principle of feminine power.
Shakti is also the name of the second of the 36 tattvas of Kashmir Shaivism. In this sense, she represents the power or energy of Shiva.
Finally, shakti can also refer to the female practitioner of Tantra, the consort of the male tantric.
In The Open Door
On Shakti by Dhruvanath