Yoga is not easy to define. In most general terms, the Sanskrit word comes from the root YUJ which means to bind or to unite, and thus stands for "union with the Self" or "spiritual discipline". What must be harnessed or mastered is attention, which ordinarily flits from object to object.
In the West, Yoga is widely practiced as a form of fitness training. But more importantly, the postures are only the "skin" of Yoga. Hidden behind them are the "flesh and blood" of breath control and mental techniques, as well as moral practices that correspond to the skeletal structure of the body.
"The fundamental objective of Hatha-Yoga or "forceful Yoga" is to transcend the egoic consciousness and to realize the Self. It is particularly focused on developing the body's potential, aiming at liberation from suffering through physical transformation." Georg Feuerstein