The Divine Feminine
Tantrics see the Divine in all aspects of the Universe. They understand the Universe, and therefore the Divine, in terms of interacting opposites - yin/yang, active/passive, emissive/receptive, masculine/feminine, powerful/motionless, and so on.
The Divine Feminine is a representation of one side of each of the infinite number of pairs of opposites. The Divine Feminine cannot exist without the other poles, so the existence of the Divine Feminine requires, also, the existence of the Divine Masculine.
The separation of the Divine into pairs of opposing forces is the first step of illusion, underlying the creation of all the other layers of illusion which prevent us from perceiving the Universe as it truly is - a unified whole.
The sexual union of the Divine Feminine and the Divine Masculine represents the underlying oneness of the Universe.
Dakini: The Divine Feminine in Tibetan Tantric Buddhism
The dakini, a female deity, has a number of symbolic meanings in Tibetan Buddhism. At the ultimate level, "she" is beyond gender, representing Absolute Insight, Universal Consciousness. She is, in essence, the vast and limitless emptiness, the lack of inherent existence of all phenomena, expressed symbolically as space, that which cannot be symbolised. However, she also manifests in a female gendered form, creating experiences in meditation, the awareness of the vital breath in the subtle body, and in the experience of transmission from a Tantric guru. Specifically, she is the inner spiritual subjectivity of the practitioner (male and female alike), the knowing dimension of experience. The encounter with the dakini is the encounter with the spiritual treasury of Buddhism,the experience of the ultimate nature of the mind in its dynamic expression as a constantly moving, sky-dancing woman. 
- Dakini's Warm Breath by Judith Simmer-Brown, Shambala (2003)