PC Booter is a general term for software that runs on early IBM PCs that require you to booted directly into the program rather than first booting into the operating system. This was done to have access to precious computer resources that would otherwise be used by the OS. Many games produced for IBM platforms in the early 1980s were PC Booters.
Although many non-IBM platforms supported booting from a disk (Apple II, Commodore 64, etc.), this was the expected behavior for those platforms, so a special term is not needed to distinguish software on those platforms.
I never owned an IBM or compatible PC where boot loading was common, but I have used them, and many IBM compatible PCs continued to support boot loading long after it was necessary.