The word zenith means peak or place close to the heavens, and so Zenyths were named after it.
Back when they were originally invented, at a date lost to time and war, the term Zenyth had a more concrete definition, however, with millennia of progress and advancement, the term Zenyth has become a catch-all for any piloted craft with limbs. Zenyths can serve many purposes, from exploration to combat. They are typically controlled through a combination of mechanical buttons/levers (think Titanfall switch heavy cockpits mixed with Pacific Rim full-body very heavy, mechanical connections), and neural connection to the pilot, through an E.L.I.T.R.A., though that exact controls will be different for every Zenyth, due to different body requirements and technological levels.
In the current day, Zenyths are separated into two main categories, Pre-War Zenyths and Post-War Zenyths. As the names imply, the largest difference is the time period they were manufactured in, altho the titles are a bit misleading, as both were used in the war it's referring to. That war is The End-Times War, where decades of tension between the human countries and empires and Corporations came to a head in the breakout of a war that lasted longer than anyone who was there at the start of it. That war, however, is not the point of this section, so go read it's article if you are interested.
The reason two distinct categories formed was due to a duo of inventions, the Atlas Connection Joint (typically referred to as "Atlas Points"), and the Gravity Shield. Before the invention of the Atlas Points, Zenyths were made as one, complex, machine, like a car today (most people just buy one completed car, rather than buying parts then assembling it themselves). This meant the cost of a Zenyth was very high, and their designs were uniform with itself. This interconnectivity allowed for extremely optimised designs, allowing a lot of firepower and strength to be packed into a relatively small frame. This interconnectivity was also, however, a detriment to their lifespan, as any damage it sustained would both be very complex and time-consuming to fix, and thus, expensive. Combined with the rapid development of weapons, making any armor rapidly proving beneficent, caused their numbers to rapidly start dwindling during The End-Times War. War started shifting to long-range void combat, with swarms of small craft with only a single large weapon, hoping to make it as hard as possible to hit them, as no armor was able to stop the new weapons. Attempting to shift the balance back toward Zenyths, the company Vespa invented the Atlas Connection Joint. While this invention gave a little bit of fighting ground back to Zenyths, it didn't do enough to save the Zenyth from losing its place on the battlefield. The invention that put Zeniths back on the map was the Gravity Shield.
Armed with Gravity Shields, Zenyths (and larger craft in general) was able to take back the throne of warfare. By this point, The End-Times War was over the half-way point, and resources, specifically, people, were running out. Due to this, most of the factories that produced Pre-War Zenyths were either already torn apart for the parts inside, or didn't have enough people to properly start production again. Due to this, Post-War Zenyths became the mass-produced Zenyth, as it is much easier for a factory to produce a single pair of arms or a cannon rather than a whole Zenyth. In the modern day, Pre-War Zenyths are very sought-after, as they are extremely rare, and much more powerful due to their all-in-one designs, and Gravity Shields make their fragility a non-factor. While there are several Atlas Point compatible Zenyth parts still made throughout the galaxy, the newer ones are even less capable compared to the parts produced during The War, due to how much knowledge was lost and how many more resources humans had access to before The Long Quiet.
Zenyths are classified by how much power they can generate/use at maximum capacity, if those two measurements are different, use the lower one for classification. There are 9 classifications, each being named after a planet in the Solar System (Most people believe that humans originated in a solar system named The Solar System, specificaly on a planet called Earth, however, the knowlege of whether this planet even existed has been lost.) in ascending order from least to greatest distance to the system's sun, ending at Neptune, at which point class ranking definitions become less standardized. Each classification requires double the power generation/use as the previous classification
I want to add some images of Zenyths here, however I don't have any yet as I would be the one drawing them (by hand, no AI!), so just give me some time.