It seems to me that nowadays the more proper term is "Trans-Neptunian Object". Is there any difference between Kuiper Belt Objects and Trans-Neptunian Objects? If not, is one term preferred over the other?
2 Answers
Yes there is a difference, Kuiper Belt Objects (KBOs) are a subset of Trans Neptunian Objects (TNOs). Other subsets are Oort-cloud objects (OCOs) and scattered disk objects (SDOs). These are not KBOs but they are TNOs. See for instance this wikipedia page.
So to refer to objects beyond Neptune you use 'trans-Neptunian objects'. It really sounds simple, there are several sub-sets of trans-Neptunian objects: referring to minor-planets (neither Planets nor comets) that exist in this region of the Solar System.