Asana (the third “limb” of Ashtanga Yoga outlined by the sage Patanjali) means to be seated in a firm, pleasant, and relaxed position. (1) Paramhansa Yogananda said that this means to keep the spine straight and the body relaxed.
It is also a term commonly used to refer to a hatha yoga postures. Yoga postures, while having many great benefits (2), is not what Patanjali was referring to in his sutras. The purpose of asana is to enable one to rise above body-consciousness. A straight spine is necessary, for when meditating the energy must be allowed to rise up the spine freely. One may consider that they have mastered asana if they can remain completely motionless, with a straight, upright spine for three hours. Motionlessness will enable them to become aware of the inner energies in the body.(1)