Apex vs. Zenith — What's the Difference?

A system of reduced fares for scheduled airline flights and railway journeys which must be booked and paid for before a certain period in advance of departure

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir).

Reach a high point or climax

The time at which something is most powerful or successful

Turn (a corner) very close to the edge of the track

The point in the sky or celestial sphere directly above an observer.

The highest point of a structure, object, or geometric figure

The point on the celestial sphere that is directly above the observer.

The usually pointed end of an object; the tip

The upper region of the sky.

The highest level or degree that is attained, as in a hierarchy.

The highest point above the observer's horizon attained by a celestial body.

The period of greatest achievement

The point of culmination; the peak

The highest or the greatest part of something, especially forming a point.

(astronomy) The point in the sky vertically above a given position or observer; the point in the celestial sphere opposite the nadir.

(geometry) The highest point in a plane or solid figure, relative to a base line or plane.

(astronomy) The highest point in the sky reached by a celestial body.

The pointed fine end of something.

(by extension) Highest point or state; peak.

(botany) The end of a leaf, petal or similar organ opposed to the end where it is attached to its support.

That point in the visible celestial hemisphere which is vertical to the spectator; the point of the heavens directly overhead; - opposed to nadir.

(botany) The growing point of a shoot.

Hence, figuratively, the point of culmination; the greatest height; the height of success or prosperity.

(astronomy) The point on the celestial sphere toward which the Sun appears to move relative to nearby stars.

The point above the observer that is directly opposite the nadir on the imaginary sphere against which celestial bodies appear to be projected

(physics) The lowest point on a pendant drop of a liquid.

The end or edge of a vein nearest the surface.

(typography):

(figuratively) The moment of greatest success, expansion, etc.

The top of the food chain.

A conical priest cap.

The tip, top, point, or angular summit of anything; as, the apex of a mountain, spire, or cone; the apex, or tip, of a leaf.

The end or edge of a vein nearest the surface.

The highest point (of something);

The point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars