A yogi is a practitioner of yoga in the disciplinary sense, one who has undertaken a systematic training of body, breath and mind directed toward liberation.
The discipline
The yogi is defined by practice rather than by office: the systematic training of the body through posture, of the breath through control, and of the mind through concentration and meditation, aimed in the classical systems at the stilling of the mind and the realisation of the true self or at union with the divine. The word covers a wide range, from the ascetic who has renounced the world for intensive practice to any serious practitioner of the disciplines of yoga, and the corresponding term for a woman is yogini.
From asceticism to the modern world
Historically the yogi was often an ascetic, sometimes belonging to orders such as the Nath yogis known for austere and physical disciplines, and the figure of the yogi with extraordinary powers gained through practice is old in Indian tradition. In the modern period yoga has been widely transmitted as a practice of health and meditation across the world, so the word now spans the traditional ascetic adept and the contemporary practitioner, and an honest account keeps the range in view. What unites the uses is the core sense of one who follows a disciplined training of body and mind, and the office, such as it is, rests on that practice rather than on any conferred rank.