A released OS. (Since: 0.2.9)
A rolling version of an OS and thus with no actual releases. For example, Gentoo, Arch Linux, etc. (Since: 1.4.0)
A continuous integration snapshot and not an actual released product yet. For example, gnome-continuous images for development snapshots of GNOME and nightly build ISOs of Fedora etc. (Since: 0.2.9)
A pre-release. For example, alpha and beta pre-releases of Fedora etc. (Since: 0.2.9)