Maithuna = Sexual Union
Maithuna or Mithuna is a Sanskrit term used in Tantra most often translated as sexual union in a ritual context. It is the most important of the five makara and constitutes the main part of the Grand Ritual of Tantra variously known as Panchamakara, Panchatattva, and Tattva Chakra.
Although some writers, sects and schools e. g. Yogananda consider this to be a purely mental and symbolic act, a look at different variations (and translations) of the word maithuna clearly shows that it refers to male-female couples and their union in the physical, sexual sense and is synonymous with kriya nishpatti (mature cleansing). Just like neither spirit nor matter by itself is effective but both working together bring harmony so is maithuna effective only then when the union is consecrated. The couple become for the time being divine: she is Shakti and he is Shakta. The scriptures warn that unless this spiritual transformation occurs the union is carnal and sinful.
Yet, it is possible to experience a form of maithuna without physical union. The act can exist on a metaphysical plane without sexual penetration, in which the shakti and shakta transfer energy through their subtle bodies alone. It is when this transfer of energy occurs that the couple, incarnated as goddess and god via diminished egos, confronts ultimate reality and experiences bliss through union of the subtle bodies.
http://en.wikipedia.org/wiki/Maithuna