Asana is traditionally defined as a “seat.” Most commonly, it is the seated posture used for meditation. More typically now 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, following after the yamas and niyamas.
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.
There are many benefits of asana practice in general. Asanas help increase flexibility and strength. They stimulate all of the physiological systems of the body, including the circulatory, immune and digestive systems. They help develop the mindfulness and focus needed for meditation. On a more subtle level, they are said to stimulate the energetic body, opening the chakras and the nadis.