Yoga is simply defined as the science of stretching to feel better. It is a Sanskrit word which means union: the union of the body, mind and spirit.
Its physical postures or asanas were developed 2000-3000 years ago by Patanjali, an East Indian philosopher, grammarian, physician and writer. Patanjali designed the asanas to stimulate or relax every aspect of the human body. Yoga assists people of all ages to improve their psychological, emotional and physical wellness, through breath, posture and movement.
Other /More definition:
yoga refer to Hindu teachings dealing with ethics, life-style, body postures, breath control, intellectual study and meditation.