Vajrasana is a very popular sitting posture in Hatha Yoga. The word vajra has several meanings, such as hard, hardy, diamond, thunderbolt, and in the context of Tantra and Yoga, the male generative organ. In Gheranda Samhita (II.12), the vajrasana posture is described as : “Arranging the thighs like a vajra, the feet are place on the two sides of the anus (gudapasv). This is Vajrasana, which yields yogic powers (siddhidayakam) to the adepts.
Traditionally and popularly, the technique of Vajrasana seems to be based on this description in Gheranda Samhita.
But in some texts, Vajrasana appears to be a name given to a famous posture called Siddhasana, literally, the adept pose.
For example, we have Vajrasana described in Yoga Kundalini Upanishad (I.6) as “putting the left heel below the kanda and the other heel on it, put the neck, head and back erect in one line.” This description is somewhat similar to the one of Siddhasana as give in the Hatha Yoga Pradipika (I.36). It is explicitly stated in the verse following this description (I.37) that this variety of Siddhasana is called Vajrasana by some experts, and some even call it Muktasana or Guptasana.
Jnanadeva, the famous adept (13th century CE),in his commentary of the Bhagavad Gita named Jnaneshwari (VI 192-199), gives the description of a sitting posture for meditation, which tallies with the description of Siddhasana and is named as Vajrasana.
Popularly, however, Vajrasana, is placing the knees together in front, the feet upturned behind the hips with hands on the knees, and head, neck and back erect.