In the first sense, Yoga means all kinds of unions. In India’s philosophical and religious systems it is used in the sense of making the union of the spiritual principle of man (Atma) with divinity (Brahma), but it also means the conscious union of the material aspect of man with his spiritual aspect, and of the superficial personality with the deep personality. In other words it is the state of integration, of conscious unification of the superficial, deep and higher aspects of man.
In the second sense, Yoga is the set of precise and systematic techniques that lead to the development of certain levels of consciousness, not updated in most men, and the integration of such levels into a single unit of total and permanent consciousness.
You will find much more about “yoga” in the Knowledge section.