These days Yoga has become known as postural exercise. But the original meaning of Yoga is quite different.
Vyasa, the foremost commentator on the Yoga Sutras of Patanjali, indicated that Yoga is samadhi. In other words, what we call Yoga is the deepest state of mental focus in which the mind is so concentrated that all one is aware of is the object of concentration and nothing else. In this state, the entire mind flows onto a single point without wavering.
Swami Rama, the great Himalayan master, also said that Yoga is samadhi. He went on to say that all the limbs of Ashtanga Yoga that precede samadhi are simply preparations for samadhi/yoga. This means that the postures, breathing practices, concentrations and meditation -- all of what we call Yoga, are not Yoga at all. Rather, Yoga is the state of oneness of Samadhi. Period.
This realization is very humbling.