Sutra 46 of the second chapter of the Yoga Sutras of Patanjali, listed in the title of this post, defines Asana, as a constituent of Yoga, as that which is stable and easeful and comfortable.
The word asana comes from the root "as," which means "to sit."
Traditionally, the word asana has three meanings:
1) A body position which is stable, motionless, without trembling, free of involuntary movements such as swaying and shifting about, easeful and comfortable.
2) A mat upon which one sits.
3) The process one goes through to sit.
This sutra applies to both postures in which one sits for meditation, and to other hatha yoga postures. The rule is that to be considered an asana, the posture must be stable, easeful and comfortable. If not, then it's not a Yoga asana.
Mastering a posture so that it is stable and comfortable requires purifying the emotions through the practice of the Yamas and Niyamas. Otherwise, emotions and mental excitement will preclude being still in a posture.