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Ubuntu JeOS (pronounced "juice") is a variant of Ubuntu that is described as "an efficient variant configured specifically for virtual appliances". It is a concept for what an operating system should look like in the context of a virtual appliance. JeOS stands for "Just enough Operating System."
Its first release was Ubuntu JeOS 7.10 (Gutsy Gibbon), and since the release of Ubuntu 8.10 it has been included as an option as part of the standard Ubuntu Server Edition.