What is yoga?

It's a practice by the means of which a spiritual seeker strives 1) to control prakriti (nature) to make the jiva (the soul) fit for union with purusha (the Oversoul), and 2) to attain nirvikalpa samadhi (union with God) and thus jivanmukta (the liberation of the soul from the rounds of birth and death).

Afterwards, the yogi is said to be a jivanmukti or atmajnani (a possessor of Self-knowledge). Western yogis prefer to call the goal "God-consciousness" or "Self-realization." Some call it "Christ-consciousness."

When did the practice begin?

Evidence of the practice, say Hindu scholars, appears in the Upanishads (ca. 1000 B.C.), which declare that atmajnana is the goal of life. Further evidence appears in the Bhagavad Gita (ca. 500-400 B.C.), which advises serious seekers of God to practice "control of the self by the Self." But yoga was officially systematized by Patanjali, a student of the Samkhya philosophy, in his Yoga Sutras (ca. A.D. 150).

Is yoga a religious or spiritual practice?

Unquestionably, yes, as its history, methods, and goal prove. Four main yogas now exist. Depending on temperament and attainments, the yogi may choose one or more of "paths" to liberation: karma (work), bhakti (devotion), raja (meditation), jnana (atmajnana, Self-knowledge: Atman = Brahman).

Ironically, hatha yoga is the third of eight limbs in Patanjali's system of raja yoga, the adoption of which presupposes the completion of the first two: scriptural study, moral purity, non-violence, etc. To practice hatha yoga is to accept the doctrines of raja yoga, involving the coiled serpent-power at the base of the spine (kundalini), seven chakras, postures (asana), breath control (pranayama), and meditation (dharana and dhyana).