The word "yoga" is derived from the the Sanskrit root yuj which means "to join" or "to yoke"; the related meaning is "to focus attention on" or "to use". In philosophical term, the union of the individual self,  jivatma, with the universal self, paramatma, is yoga.

The union results in a pure and perfect state of consciousness in which the feeling of "I" simply does not exist. Prior to this union is the union of the body with the mind, and the mind with the self. Yoga is thus a dymanic, internal experience which integrates the body, the senses, the mind, and the intelligence, with the self.

From B.K.S IYENGER – YOGA – the path to holistic health page 46