The first teaching that Krishna gives in The Bhavagad-gita is how to act, how to use your skill, your energy, your body, to act; this is called karma yoga, which is simply how to behave, how to work, how to be engaged in the world, how to take care of responsibilities, how to perform any action — even mentally or emotionally, not just with the hands, the body.
Yoga Defined
(Sanskrit योग) “union.” Similar to the Latin “religare,” the root of the word “religion.” In Tibetan, it is “rnal-‘byor” which means “union with the fundamental nature of reality.”
Yoga in a generic sense refers to karma yoga, bhakti yoga, raja yoga, jnana yoga, hatha yoga, mantra yoga, laya yoga, or kundalini yoga. In a restricted sense, it means the ashtanga yoga or raja yoga of Patanjali Maharshi.
The word yoga means in general to join one’s mind with an actual fact. The 14th Dalai Lama
The word yoga comes from the root Yuj which means to join, and in its spiritual sense, it is that process by which the human spirit is brought into near and conscious communion with, or is merged in, the Divine Spirit, according as the nature of the human spirit is held to be separate from (Dvaita, Visishtadvaita) or one with (Advaita) the Divine Spirit. Swami Sivananda, Kundalini Yoga
Patanjali defines Yoga as the suspension of all the functions of the mind. As such, any book on Yoga which does not deal with these three aspects of the subject, viz., mind, its functions and the method of suspending them, can be safely laid aside as unreliable and incomplete. Swami Sivananda, Practical Lessons in Yoga