I think this piece by Dr. David Simon exquisitely sums up what Asana is:
“Asanas are usually defined as the various yogic postures designed to bring balance
and harmony to the physical body, particularly the musculo-skeletal system. Asana
is part of the Ayurvedic treatment system for the physical body.
At a deeper level, asana refers to the complete expression of mind-body integration, a state in
which we become conscious of the flow of prana resonating in every molecule of our body and
in every thought and every experience.
The essence of asana isn’t about straining to get our body into a particular posture; it’s about
surrender, opening, expanding, and enhancing our flexibility, balance, and strength. The
practice of asanas is a way to expand our own sense of self and experience the joy of being
incarnated in this physical body. At the same time, it allows us to create an intimate dance
between our individuality and universality and celebrate the essence of this connection.”