The first sutra and the definition of yoga according to maharshi Patanjali is

atha yoga anushasanam

If we bifurcate the sutra.

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    = of yoga, 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

Samadhi pada 1.1

As Śrī Maharshi Patañjali began by the term atha which means now or at this time, I will do something or I need to start something.

He took the task of giving the explanation of yoga and its practice because before this point such an explanation was not laid out in an academic way. He gave the syllabus for yoga, thus breaking the monopoly of all those teachers who mastered yoga and who taught it to their students’ bit by bit over the years.

Whatever Śrī Patañjali would say would be standard. It cannot be changed merely by a difference in philosophy.

Similarly, as a gas-burning motor made in Japan will be almost the same as one produced in Germany, so yoga practice will be the same everywhere because the human body is the same in each case and the way of changing the subtle form which produced that gross one is also the same.

Bhoja, one among the classical commentators on these aphorisms, defines Patanjali’s use of the word yoga as “an effort to separate the Atman (the Reality) from the non-Atman (the apparent).”

To summarize the first sutra,

Now, after having done prior preparation through life and other practices, the study and practice of Yoga begins.

One who practices yoga is called a yogi.

Thank you