Asana is the third limb of Patanjali’s Eight Limbed Path as described in the Yoga Sutras, which in turn is part of the philosophy of Raja Yoga.

sthira sukham asanam

“sitting posture should be steady and pleasant”

sthira = strong, steady, stable, motionless, solid, alert
sukha = comfortable, relaxed, at ease
asanam = posture, seated position, physical practice

Asanam means two things:
1. The place on which the yogi sits
2. The manner in which one sits there

Every asana is practiced with both sthira and sukha. There is no asana without these qualities.

The sitting place should be steady, neither too high nor too low, and situated in a clean spot. Sitting posture should be steady and straight, holding the spinal column, neck and head in a straight line but without strain; so that one can forget the body altogether. Thus the asana that brings about a state of equilibrium in the body should be practiced.

We might come into a posture that feels tense and then our mind is concentrated only on the tension. In this case, we are not really in the asana. We should start practicing easy postures and we progressively achieve steadiness and overall comfort. Perhaps we sit comfortably in a posture but our mind is completely somewhere else. That is not asana either. The posture which is steady and comfortable is the asana.

Asanas as we know them in modern times are meant to relax the body and to activate and strengthen various organs in the body. As the body becomes relaxed, one’s mind is viable for meditation without any distraction caused by physical discomfort.

During the yogic practice the regular nerve currents are displaced and new channels will begin. New subtle vibrations start along the spinal column and will remodel the entire body. As the main part of yogic activity takes place in the spinal column, it is necessary that the spine must be kept in straight line. Therefore, the waist, chest, neck and head are kept in a straight line. This posture facilitates free flow of pranic energy in the spine. By the practice of correct posture, physiological, psychological and spiritual benefits are derived. The object of asana practice is to eliminate body consciousness. If one is disturbed by body consciousness of any kind, one cannot concentrate the mind.

Asana is sometimes categorized under Hatha Yoga. The postures should be designed in a such way to massage the endocrine system and realign the subtle energy channels of the body, releasing blockages and increasing Prana. This allows a meditative state to be brought about. The body and mind should remain one-pointed.