famous sūtra of Patañjali (Yogasūtra 2- 46 )
Sthira sukham āsanam
describing how a Yoga posture should be, its quality. “Sthira”, see above. “sukham” means comfort, ease, filled, accomplished, enjoyable and “āsanam” means posture, position, seating, hence the translation of this sutra as “The position is (should be) firm and pleasant”.