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 surviving/existing as a basic conditioned animal, 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 human can strive to perfect one or more of the different areas of their development in order to become more than just a simple animal, but it is only when all eight possible areas of development are perfected that one truly graduates to reaching their highest potential.
So, What is Yoga?