Commentary from Light on the Yoga Sutras by B.K.S. Iyengar
Asana is perfect firmness of body, steadiness of intelligence and benevolence of spirit.
Sutras 11.46-48 define asana and the effects of its practice.
The definition of asana is given as follows: whatever asana is performed, it should be done with a feeling of firmness, steadiness, and endurance in the body; goodwill in the intelligence of the head, and awareness and delight in the intelligence of the heart. This is how each asana should be understood, practiced, and experienced. Performance of the asana should be nourishing and illuminative. * Please read more in Light on Yoga*