The word yoga has many different meanings, both in Sanskrit and in English. I will use it in this text mostly for the entire group of spiritual paths developed in India to attain knowledge of the Self, liberation from ego, and transformation of one's nature. Unless specifically mentioned otherwise, I will not use it for asanas (hathayoga postures), not as a shortcut for the darshana (philosophical school) called Yoga, nor for Kriyayoga, Patanjali's Rajayoga, or any other specialised path.
Sticking closely to the original Sanskrit meaning of the word, Sri Aurobindo defines it in The Synthesis of Yoga (p. 32) as follows:
Yoga is the union of that which has become separated in the play of the universe with its own true self, origin and universality.