When it comes to Asanas, the definition is very simple, Asana is a posture, of course, but which type of posture? It is a posture where you are steady and comfortable, happy, and peaceful. If you are uncomfortable, if you are unsteady in a posture, then that would not be labelled as an Asana. Any fast movements would not be recommended nor considered as Asana.
But the question is, if the body is stiff or sick, any posture could be uncomfortable for that person. So for such conditions, we have a few derivatives out of the main Asanas to help that person; to add more flexibility, to see that muscles are toned up in these areas and so on. It will do some of these very simple derivatives of the main Asanas and understand their effect in different areas of our body.