Actual meaning of "Yoga" is the "Union of the self (Individual Soul or Atma) with the Ultimate reality (Paramatma or God)".
Yoga is also the final goal of the followers of the "Bhagwat Gita". There are many types of Yogas explained in Hindu mythology, like "Gyan-Yoga" (doctrine of the way of knowledge) , "Karma-Yoga" (doctrine of the way of action) or "Bhakti-yoga" (doctrine of the way of devotion) etc. Each doctrine shows the way to reach God in it's own aspect. All these different types of Yogas are explained in different religious books of Hindu religion but all doctrine are scattered in different books.
But in "Bhagwat Gita" there is reconciliation of all these different doctrines at one place. There are different chapters in Bhagwat Gita which are dedicated to each of these doctrines. So it is called that if a person wants to get all the different knowledge of Vedas & Puranas, then he/she can get all these in one place at Bhagwat Gita. So it is good for a person to read all these doctrines in Bhagwat Gita than searching for it in different books.
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