The Yoga Sutras of Patanjali is an ancient philosophical text. Sutras are short aphorisms addressing a particular topic, in this case yoga. The second sutra provides a definition of yoga: "yoga chitta vritti nirodha." Though there are variations on how this statement is translated from the Sanskrit
, one common interpretation is "yoga is the cessation of the fluctuations of the mind." In other words, you are doing yoga in order to achieve a stillness and clarity of mind.