There are many ways to define Yoga.
In essence, however, it is a lifestyle choice marked by striving to be the best human-being that one is capable of being in this lifetime.
The aim of the lifestyle/practice is to move from simply existing as a basic conditioned animal in the world, to truly living as an enlightened human-being while embodied here on Earth. And, there are many paths/steps for achieving this goal.
In one lifetime, a person can strive to perfect one, or several, of the different areas that are possible in their overall self-development, but it is only when all eight possible areas of development are perfected, according to Raja (or Royal) Yoga, that one truly graduates to reaching their highest potential.