In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers to the highest point reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is often used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.
Strictly speaking, the zenith is only approximately contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).
Okay, so in plain terms (and this definition came from that fountain of all internet knowledge...WIKIPEDIA), if something is at its zenith it's straight overhead. So I suppose that's a good choice of name for the two flagships of one of my fleets in one of my sci fi series. The Zenith and the Zenith II are Captain Jaspen's "babies". They are his whole reason for existing - or so he tells everyone. When the Zenith gets destroyed, Jaspen refuses to be relegated to a small battle cruiser. He wants his big ass ship back. So he contracts with his father - the whole family is into R&D and engineering - to make him a better fleet flagship.
He does, and Jaspen gets the Zenith II. Jaspen is thrilled with his new toy and immediately goes out into the war zone to break it in. Well, he needed to get back out there because he was the commander of the entire group of naval forces in the system. Not everyone is as pleased with the Zenith II as Jaspen is. But that's because they can only see the bugs in the systems, the glitches that you get when you've got something new and shiny that's never been used before. Jaspen, on the other hand, knows that if they just work at it they'll learn to love the ship as much as he does. He sees the potential.
Many times, as writers, we love our "babies" to death. We can see the potential. We know what's there. But we don't always see the nitpicky little details that could completely derail our stories and send them into the pile of rejection worthy material. This is what our beta readers and our editors are for. Beta readers, editors, and writing groups are going to help push you in a better direction.
Notice I didn't say "the right direction". Only you can decide what the "right direction" for your writing is. But don't ignore suggestions either. Sometimes that glitch can kill a story as quickly as losing oxygen will kill my space naval officers and crew. Focus on the big picture only and you miss out on the little details. Don't obsess over the details to the point where you lose sight of the big picture either. Find your balance and stick with it.
And never stop dreaming. Dreaming big is what writers are here to do.