zen ‧ ith /ˈzenəθ, ˈzenɪθ $ ˈziː-/ BrE AmE noun [countable usually singular]
[ Date: 1300-1400 ; Language: Old French ; Origin: cenith , from Arabic samt (ar-ra's) 'path (above the head)' ]
1 . the most successful point in the development of something SYN peak OPP nadir
reach its zenith/be at its zenith
The Roman Empire reached its zenith around the year 100.
2 . technical the highest point that is reached by the sun or the moon in the sky
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Zenith sounds rather literary. In everyday English, people usually say peak :
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Her career was at its peak.