In the beginning, yoga was passed verbally from guru to disciple. An ancient yogi named Patanjali, became the first to describe the discipline of yoga in a scientific way when he wrote The Yoga Sutras. By many, he has been called the Einstein of yoga.

In Sanskrit, the word sutra means thread or string, and so The Yoga Sutras are words “strung or bound” together to describe the complete philosophy of yoga.  He defined the sutras in four chapters Samadhi pad, Sadhana pad, Vibhuti pad and Kaivalya pad. 

“Athayoganushashnam”

Now, after having done prior preparation through life and other practices, the study and practice of Yoga begins. (atha yoga anushasanam)
Yoga Sutras 1.1 - Patanjali

atha - now, at this auspicious moment; implying the transition to this practice and pursuit, after prior preparation; implying a blessing at this moment of transition

yoga - yog, union; literally, to yoke, from the root yuj, which means to join or to integrate; same as the absorption in samadhi,

anu - within, or following tradition; implies being subsequent to something else, in this case, the prior preparation

shasanam - instruction, discipline, training, teaching, exposition, explanation; Shas implies the imparting of teaching that happens along with discipline.