An asana is a body posture, originally a sitting pose for meditation Later in hatha yoga and modern yoga asana evolved into yoga poses, adding reclining, standing, inverted, twisting, and balancing poses to the meditation seats. The Yoga Sutras of Patanjali define “asana” as “[a position that] is steady and comfortable”
Sutra 2.46: sthira sukham asanam
The posture of the body during the practice of contemplation and at other times, as also the posture of the mind (or attitude to life) should be firm and pleasant. Translation and interpretation by Swami Venkatesananda
All above photos show B.K.S. Iyengar presenting various asanas.
For more yoga poses check Light on Yoga by B.K.S. Iyengar
B.K.S. Iyengar books on Yoga Sutras:
Light on the Yoga Sutras of Patanjali
Core of the Yoga Sutras