The concept of a'Tha was a Vulcan belief that could mean Immanence in the Human language. It covered the concept of a direct experience with the guiding force that governs the universe itself. Vulcans believe that they are in direct and constant encounter with this presence in various degrees; as such it is interpreted differently by many Vulcans.
It is believed to have been the oldest Vulcan word and the first ever written. In terms of effect, it was similar to the concept of God to other races. (TOS novel: Spock's World)