The word yoga means “yoke” or “union.” In the classical sense given by Patanjali, yoga is “the cessation of the modifications of the mind” — the dissolving of the inner turbulence that obscures what one already is.
The Bhagavad Gita offers four principal yogas: Jnana Yoga (knowledge), Bhakti Yoga (devotion), Karma Yoga (selfless action), and Raja Yoga (“royal” — the meditative path systematized by Patanjali).
The eight limbs of Raja Yoga — ethics, observances, Asana, Pranayama, sense withdrawal, concentration, meditation, and Samadhi — structure most contemporary yogic practice, though modern postural yoga is only a small slice of this.