Yoga has been around for a long time, with many schools of thought and many variations. Modern Yoga Philosophy puts union out of reach of individuals, because all training comes out of a book without any actual experience. I suspect most people know the meaning of Yoga as Union. The Union of the Soul with the Body, inner nature with the physical nature.
So what is the problem, we have always been in Union with our Inner Nature.
Our Inner Nature communicates through to a us in a language that we forgot.
It screams at us for some attention, with various pains that we take a pill for or go to the Doctor. It cries out for love and appreciation and you ignore the actual thing your own Soul, that you are searching for.