zenith
[noun]
Synonyms: height, acme, apex, apogee, climax, crest, high point, peak, pinnacle, summit, top

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Antonyms: nadir

[TahlilGaran] English Synonym Dictionary ▲

zen‧ith /ˈzenəθ, ˈzenɪθ $ ˈziː-/ 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 Synonym : peak Antonym : 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

REGISTER
Zenith sounds rather literary. In everyday English, people usually say peak:
Her career was at its peak.

[TahlilGaran] Dictionary of Contemporary English ▲