Sutra 46 of the Yoga Sutras. (Sadhana Pada) Chapter on practice

Sthira Sukham Âsanam
Sthira=steady, sukham=comfortable, asanam=posture

Asana is a steady, comfortable posture.

Asana pictures
The Yoga Sutras teaches there are eight limbs in order to achieve Yoga. These limbs begin with the most outward level of our gross actions towards others and progress towards the most inner and subtle experience of complete Super Consciousness or Yoga. These Sutras are also called Ashtanga Yoga or eight limbed Yoga. We should remember that each of the eight limbs is equal to the others and necessary.

1. Yama
(abstaining from killing, lying, stealing, sexual harm, and greed)

2. Niyama
(observing purity, contentment, discipline, study, and surrender)

*3. Asana Posture

4. Pranayama (breath control)

5. Pratyahara (sense withdrawal)

6. Dharana (concentration)

7. Dhyana (meditation)

8. Samadhi (Yoga the final state of absorption)

Asana is the third limb of Yoga and the Sutra above describes Asana as sitting in a single pose in order to practice the more subtle limbs of meditation.

People trying to sit quietly in meditation found they couldn't. As soon as they sat down they encountered a small cramp here, a tiny pain there and were continuously reminded of their legs, hands, hips and spine. They encountered pain, stiffness, bile, gas, etc., and thought '"What is the reason for all these things and how can we get rid of them?" Yogis developed many preliminary poses in order to make the body more supple and free from toxins so the body could sit in meditation with comfort and ease. Asana is a limb that is helpful for achieving the final tate of Yoga or liberation.