The Gheranda Samhita describes vajrasana as setting the feet on either side of the anus and making the thighs firm, hard as a vajra. The text supplies its own comparison, which is unusual: most posture names in the corpus are left unexplained, and this one is glossed inside the description itself.
The vajra is the thunderbolt and diamond weapon, the emblem of Indra in the older Vedic material and, in Tantric Buddhism, the ritual implement whose name is carried by the whole vehicle called Vajrayana. The word means both the unbreakable and the irresistible, and it is one of the most heavily loaded objects in South Asian religious vocabulary. A posture named after it is claiming solidity rather than resemblance, which sets it apart from the animal names that dominate the same list.
The Hatha Yoga Pradipika does not carry the posture. It belongs to the group of low kneeling and squatting forms that appear only in the later manual, alongside the frog and the raised posture, and like most of that group it is described precisely and briefly, with no therapeutic claim attached. The atlas records the description the text gives and the sense the text supplies for the name.
The vajra is also a physical object in living practice. In Tibetan and Newar Buddhist ritual the implement, called dorje in Tibetan, is held in the right hand paired with a bell in the left, and the pair carries a settled doctrinal meaning about method and wisdom held together. None of that is invoked by the Gheranda Samhita, which uses the word for hardness alone, but it is why the term will be familiar to readers who have met it in an entirely different setting.