OSMedia is an abbreviation of Operating System Media. OSDBuilder defines OSMedia as an Operating System Media without customizations.
An Operating System ISO downloaded from Microsoft MVLS or MSDN would be considered OSMedia.
Microsoft releases monthly Security Updates, and sometimes releases updated OSMedia with these monthly Security Updates included. OSDBuilder "mirrors" this process to create updated OSMedia through Offline Servicing. This is not considered a customization as there are no material changes to the Operating System itself (with the exception of optionally updating OneDriveSetup.exe)