Merriam-Webster Collegiate Dictionary - zenith

 noun  Etymology: Middle English cenyth, senyth, from Middle French cenit, from Medieval Latin, from Old Spanish zenit, modification of Arabic samt (al-ra's) way (over one's head)  Date: 14th century  1. the point of the celestial sphere that is directly opposite the nadir and vertically above the observer — see azimuth illustration  2. the highest point reached in the heavens by a celestial body  3. culminating point ; acme at the ~ of his powers — John Buchan