In the Yoga Sutra, Patanjali describes asana as the third of the 8 limbs of classical, or Raja Yoga. These eight limbs are the yamas (restrictions), niyamas (observances), asanas (postures), pranayama (breath work), pratyahara (sense withdrawal or non-attachment), dharana (concentration), dhyana (meditation), and samadhi (realization of the true self and/or unity with god).
Asanas are the physical movements in a yoga practice. In combination with pranayama or breathing techniques, it constitutes hatha yoga. In the Yoga sutras, Patanjali describes "asana" simply as sitting meditation, suggesting meditation to be the path of samadhi, or transpersonal self-realization.