In yoga, the term is used for any physical posture of Hatha yoga. It is the third limb of Patanjali’s eightfold path of ashtanga, followed after the yamas and niyamas.
Asana practice is considered important by yogis because it helps to keep the physical body healthy. Given that the body is the vehicle for the spirit, looking after the physical body is important for spiritual development. Practicing asanas can also have a range of emotional and energetic benefits, increased discipline and concentration, and readying the mind for meditation.
In Patanjali’s Yoga Sutras, asana is defined as a “steady, comfortable posture.” The sutras do not specify any particular poses, but require simply that in practicing asanas, a position which is both steady and comfortable should always be sought.