The Sanskrit words Yogi (m.) and Yogini (f.) name a person who practices Yoga, or who has succeeded in it.
Within the International Nath Order Yoga is union with the Cosmic Spirit. A yogi is one at that work. A Nath or Devi of this Order may be a yogi. The words are not one. Yogi is not sadhu, and it is not Adept. Those name a renunciate life and an attainment. This page names the practitioner.