Everything in nature is conditioned by the fundamental principle expressed in the unity of opposites: yin-yang. The fusion of the male and the female is a creative act and the source of life. Even though Buddha Nature is beyond genders, Buddhist iconography uses sexual polarity to symbolize the Mahayana and Vajrayana concept of the union of principles: female wisdom (Skt. prajna, Tib. sherab) and male compassion (karuna, nyingje) or skilful method (upaya, thab).
The union of wisdom and compassion symbolizes the non-polarized state of bodhicitta (jang chub sem), or the mind of enlightenment, which is represented visually by showing two deities engaged in sexual union. In Tibetan Buddhism such images are known as yab-yum, which literally means father-mother. The Sanskrit term for such union is yuganaddha (union of opposites), which refers to Tibetan term zung jug.