"kaya sosanam tapah" - Tapas is the detoxification [process] of the body.
The word tapas is often used in Yoga circles, and is normally translated as heat. While this could be a partial meaning of the word in Sanskrit language, it needs to be interpreted differently in the context of Yoga. There is a rule in Sanskrit grammar that meanings have to be interpreted based on the context of its usage. It is using this rule that one of the commentators of the Yogasutra-s of Patanjali has presented the above definition of tapas.
The concept of tapas is presented in the second chapter of the Yogasutra-s, a chapter that is detailing the reasons of our suffering, and the method through which we can overcome them. In fact the second chapter starts with the word tapas.
The assumption is that the body has certain toxins in the body and we need to systematically overcome them through a process. And this process is tapas. The toxins that Yoga is talking about is not merely physiological. They also include mental, emotional and spiritual. And this is indeed why tapas is not merely physical practices, but also include process to refine our mind, emotions and spirit.
The very fact that we are doing tapas, through practice of any of the eight limbs, is an acknowledgement that we have toxins in one form or another. So we have to remember this, my fellow yogis, that just because we practice yoga for a long time, does not mean we are pure and perfect. Rather, we are acknowledging that we are aware of our toxins and are working on them.
Yoga teaches us that we are perfect as we are, with our defects, as long as we engage in holistic tapas to work on reducing them.