A “zenith” is the highest point. If you’re speaking of something measured in terms of time, it’s the culmination of that timeline. You’ll also hear the word used in astronomy: the zenith is the point directly above any particular location on Earth, from the perspective of someone standing on the ground looking straight overhead.

Example:  “Investment in internet start-ups during the dot-com craze reached its zenith in the late 1990s, when a new business was going public every few days.”