Dancing Shiva

What is Yoga?

The Yoga Sutras of Maharishi Patanjali provide a comprehensive answer to this question.
The 2nd Sutra of the 1st Book of The Patanjali Yoga Sutras - The Samadhi Pada - summarises the fuller exposition:
Shri Patanjali
"Yoga is the restriction of the fluctuations of consciousness." (translation by Georg Feuerstein)