"The word "hatha" may esoterically be said to derive from the Sanskrit terms "ha" meaning "sun" and "tha" meaning "moon". Thus, Hatha Yoga is known as the branch of Yoga that unites pairs of opposites referring to the positive (sun) and negative (moon) currents in the system."[1] Historically, however, the term haṭha refers to the use of persistence or force, and haṭhayoga is translated by the Monier-Williams dictionary as "a kind of forced Yoga or abstract meditation (forcing the mind to withdraw from external objects; treated of in the Haṭha‐pradīpikā by Svātmārāma and performed with much self‐torture, such as standing on one leg, holding up the arms, inhaling smoke with the head inverted &c.)."

Extracted from www.wikipedia.org.