zenith

Concise Oxford English Dictionary © 2008 Oxford University Press:
zenith /ˈzɛnɪθ/

▶noun

• 1 the time at which something is most powerful or successful.

• 2 the point in the sky directly overhead. The opposite of nadir.

  ■ the highest point in the sky reached by a given celestial object.

– derivatives
zenithal adjective.

– origin ME: from OFr. or med. L. cenit, based on Arab. samt (ar-ra's) ‘path (over the head)’.