"Yogah Karmasu Kaushalam", "Samatvam Yoga Uchyate" - These are the famous definitions of Yoga as given by Lord Sri Krishna in Bhagavadgeetha. Dexterity in action is called Yoga. Its an union of body, mind and intellect to achieve a higher spiritual growth and self realisation. Yoga stresses on having a balanced and equanimous mind at all times.
Who is a Yogi? As the name suggests, one who practice Yoga is a Yogi.