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In Patanjali's Yoga, Asana does not mean a specific posture, but means sitting. It means, mainly, sitting for meditation. Asana means a meditative seat.
Asana is the third rung in the ladder of the practice of Yoga. If the Yamas and Niyamas are the foundation of Yoga, Asana may be regarded as its threshold. 'Asana', literally, means a seat. Here 'seat' does not mean a cushion or some such thing that is spread on the ground. Asana is a pose of the body or the posture which it assumes at the commencement of the practice. It is called a 'seat', because it is a posture of sitting and not standing.
The conditions for a good Asana include:
1. The Asana should be firm and easy. It should be steady and not cause discomfort of any kind. It should not make the student conscious of the body through tightness, tension, etc. It should be a normal posture in which he can sit for a long time.
2. The Asana should be effortless. There should be no effort not only in the body but also in the mind. Absolute ease of relaxation is the sign of perfected Asana. The student should be in a most natural condition in which he is not conscious even of his breathing.
When this bodily control is achieved, there comes freedom from the onslaught of what are called the 'pairs of opposites', such as heat and cold, hunger and thirst, joy and grief, and so on.
Yoga is rhythm. Asana is therefore the beginning of Yoga, wherein one starts relating oneself to the cosmic order.
The Hatha Yoga Pradipika By Swami Svatmarama (External link) suggests the following "sitting for meditation asanas".
Other translations of the Hatha Yoga Pradipika have more to say about the sitting asanas, albeit cryptically.
Svastikasana: "Sitting straight on level ground, squeeze both feet between calves and thighs [of the opposite legs]. This is svastikasana."
HYP Chapter 3 Asanas: sutra #19
Virasana: "Place one foot upon the other thigh and the other foot below the opposite thigh. This is virasana."
HYP Chapter 3 Asanas: sutra #21
Siddhasana: "Press one heel into the place below the sex organs [the perineum] and put the other heel just above this region [close to the abdomen]. Press the chin upon the chest, sit up straight, with controlled organs, and fasten the eyes between the eyebrows. This is siddhasana, whereby all obstacles on the path to perfection are removed"
HYP Chapter 3 Asanas: sutra #35
Vajrasana: "Place the right heel above the sex organ and the left heel over the right. This too is siddhasana."
HYP Chapter 3 Asanas: sutra #36 "Some call this siddhasana; others say it is vajrasana, or muk-tasana, or guptasana"
HYP Chapter 3 Asanas: sutra #37
Padmasana: "Place the right heel upon the base of the left thigh and the left upon the right thigh. Cross the arms behind the back and grasp the toes, the right ones with the right hand and the left with the left. Press the chin on the breast and look at the tip of your nose. This is called padmasana and cures all diseases."
(The secret teaching is that there should be a space of four inches between the chin and the breast" Sri Nivasa lyangar. The Hatha Yoga Pradipika of Yoga Swami Svatmarama (translation with commentary) (Adyar, 1949), p. 22. -Trans.)
HYP Chapter 3 Asanas: sutra #37
