Container OS or Base OS refers to an image that contains an operating system such as Ubuntu, CentOS, or windowsservercore. Typically, the local image is built on top of a Base OS image so that it can take use parts of the OS. Note that windows containers require a Base OS, while Linux containers do not.
Source: http://www.floydhilton.com/docker/2017/03/31/Docker-ContainerHost-vs-ContainerOS-Linux-Windows.html