Hatha Yoga combines the Sanskrit words ha, meaning sun, and tha, meaning moon.  Together hatha means “force.”The word yoga means union.  So Hatha Yoga is the yoga of force.  It is the practicing of balancing opposing energies of the body: sun and moon, male and female, shiva and shakti, left and right, hot and cold.  Hatha yoga says that we unify these energies with physical postures, or asanas, breathing, or pranayama, and meditation.  The idea is to still the mind and create balance, to purify and activate the energy centers of the body.