Yoga is a powerful interconnection between

• individual consciousness and cosmic consciousness
• individual soul and universal soul
• a being; and God
• the cores; of all the beings
• aspirations of all the beings
• the ‘cause’ of the universe and the universe
• the ‘cause’ of the past, present and future; and rest of the time, space, energy, matter and consciousness
• the contingent and the field
• spirit (soul), intelligence, emotions, instincts and metabolic activities in different systems of body
• holistic perspective, thinking, emotions, instincts and actions
• cognition, affect and conation
• spiritual, cognitive, affective, psychomotor and productive domains
• holistic approach; and analytical, synthetic, reductionist and other approaches
• holistic activities aimed at holistic health; in all the fields of life
• two sides of brain
• reason and intuition
• cerebral cortex, limbic system, autonomic nervous system, neuroendocrine system, endocrine system and the metabolic activities
• immortalizing ocean of ambrosia viz. Nama; and the process of Namasmaran going on in billions of individuals

This is only a frail attempt of its definition; as Yoga is beyond definition and description