One who seeks union with God, or one who practices yoga. These designations are mostly reserved for advanced practitioners. The word "yoga" itself—from the Sanskrit root yuj ("to yoke") --is generally translated as "union" or "integration" and may be understood as union with the Divine, or integration of body, mind, and spirit.
A yogi is a practitioner of yoga. The term "yogi" is also used to refer specifically to Siddhas, and broadly to refer to ascetic practitioners of meditation in a number of Indian religions including Hinduism, Buddhism, and Jainism.
Example via www.ramdass.org: “Autobiography of a Yogi” by Paramahansa Yogananda