Cosmology is defined as the study of the origin, evolution, and fate of the Universe. This can also be defined as physical cosmology, contrasting to religious cosmology which is a set of beliefs about spiritual realms as well as the origin of the Universe.
In this wiki, as well as in popular terms, it is also defined as the study of, as well as the organizational structures of Reality or fiction that are at the size of or larger than a Universe. Is there more than one Universe and if so how do the multiple universes relate to each other, what larger structures do they form and how are they organized? This is currently just a matter of speculation and everyone is free to define their own cosmology as per The Law of Cosmological Subjectivity. This speculative activity is exactly the point of this wiki.
Despite it being a speculative activity, one can say that there is a real cosmology. We just don't know how it looks like. It may just be the case that the real cosmology is just the single universe we live in, without nothing else beyond it. Although there is no real proof of anything more, the belief in the existence of something larger is a common one with several hypotheses having been proposed in the scientific discipline of theoretical physics.
As it refers to elements larger than a universe, things like solar systems, galaxies, clusters, and superclusters aren't exactly what it is about. Those things are better referred to as astronomical elements, studied by astronomy, and they exist at a smaller scale than those the term cosmology is used to refer to. One can imagine any type of structure at the cosmological level but there are recurring elements that are traditionally used and commonly accepted and understood. The most basic of those and the starting point is the Universe, allowing for other types of universes, more or less similar to our own, of which many were speculated upon. Then comes the Multiverse, typically a group of universes, possibly infinite in size. Many much larger structures have also been speculated and can be found throughout the wiki, some more consensual, others less so. Another typical structure is the Omniverse which usually refers to the entire set of all reality. Even larger structures than that, transcending actually reality, have been speculated upon and some are subject to much debate. In more abstract contexts, however, cosmologies can be considered as anything, whether it exists or doesn't exist.
Despite sharing many similarities in both their organizing structure and common elements and/or names, most cosmologies are independent and unrelated to each other. The idea that they could all describe the same thing from different points of view as also been purposed.
A list with some of the cosmologies described on this wiki and their containing elements can be found on the Cosmology Tier page.
Most cosmologies imagined end up sharing the same top level structure in one way or another. This ends up being something more akin to a meta-structure used to given a common base to all cosmologies. They are then viewed as different areas existing inside such structure at the same time. Under the view of the Box for example, each different cosmology will probably exist in its own Altarca. Not everybody shares this view though.