Short Definition
A system of exercises for attaining bodily or mental control and well-being. Various forms of yoga include poses (or asanas) for building strength and flexibility, breathing exercises for cleansing, and/or meditation for relaxation and stress reduction.

Expanded Information
Yoga is one of the most ancient cultural heritage of India. The word yoga in Sanskrit means "to unite", and so yoga can be said to connote a unitive discipline. In this sense it is an exercise in moral and mental cultivation that generates good health (arogya), contributes to longevity (chirayu), and the total intrinsic discipline culminates into positive and perennial happiness and peace. Therefore, yoga is the said to be indispensable of the ultimate accomplishment in life. It is a science that affects not only the conscious self but the subconscious as well. It is a practical physiological training (kriya yoga), which if practiced can exalt man to the 'supra mundane level'.

What Yoga Is Not
There are too many misconceptions clouding the science of Yoga. People perceive it to be some kind of black or white magic, sorcery, physical or mental debauchery through which miraculous feats can be performed. For some it is an extremely dangerous practice which should be limited to only those who have renounced the world. Few others think it to be a kind of mental and physical acrobatism that is compatible only to a Hindu mind.

What Yoga Really Is
Yoga is an all-embracing way of life, a science of self-culture and mental discipline that ensures the purgation of the ignoble in man and brings forth what is most noble in him. It is pertinent to all people irrespective of his caste, creed, sex, and religion. It can be beneficial to all - the good and the bad, the sick and the healthy, the believer and the non-believer, the literate and the ignorant, the young and the old.

The Origin of Yoga
Yoga had its genesis in the wandering ascetics who sought the solitude of the forests to practice this ancient science and then imparted their knowledge to the ardent students (mumuksu) who lived in their ashrams. The ancient yogins were possessive about this art form and did not make any effort to popularize yoga. The yogic postures and the subsequent stages of yoga were handed down only to the deserving students. Hence, this science remained limited to the confines of the forests or remote caves.Very little was known about this Vedic practice until the Yoga Institute of Santa Cruz, Mumbai was founded in 1918, which became India's oldest technical institute on Yoga.


Source: http://www.americanyogaassociation.org/