Definition - What does Tha mean?
Tha is a Sanskrit word that translates as “protector,” “preservation,” “fear” and “mountain.” In the modern world of Hatha yoga, tha has come to symbolically represent the moon. More specifically, the word, hatha, is represented by ha for the sun and tha for the moon.
Sanskrit scholars argue that these translations are not correct and that "ha" and "tha" are simply syllable sounds in the context of hatha. Even so, the idea of sun and moon has become so prevalent that it has come to represent the practice of Hatha yoga as a balance between the physical body and mind/spirit.
Yogapedia explains Tha
The origins of tha as the moon seem to date back roughly 500 years to Swami Svatmarama and his manual, "Hatha Yoga Pradipika," in which he outlined the main practices of Hatha yoga. He described the two major energy channels in the body, pingala and the ida and compared them to opposites like hot and cold, solar and lunar. Through Hatha yoga, these channels can be united. Referring to tha as the moon, therefore, was an analogy rather than a literal translation of part of the word hatha. The actual translation of hatha is “force,” “exertion” or “effort.”
Following the modern symbolism, tha is associated with the ida nadi (energy channel of the body). This is the lunar channel, which is the cooling, calming energy that stills the mind and relaxes the body. It controls the creative side of the brain.