Center Of Life

Riddle: There is one word that stands the test of time and holds fast to the center of everything. Though everyone will try at least once in their life to move around this word, but in fact, unknowingly, they use it every moment of the day. Young or old, awake or in sleep, human or animal, this word stands fast. It belongs to everyone, to all living things, but no one can master it. The word is?