Just enough Operating System, JeOS, pronounced like Juice. Is a term used to describe an operating system that has been stripped down from unecessary packages. It is a bare bones system for all intents and purposes. Some of the JeOS releases have optimized kernels for various virtualization platforms making this type of distro favorable if you have a virtual environment like Xen.
Many people believe that the JeOS term is just a hype name for what has been known as a barebones installation for a long time. I have to admit that it is nice to have a really small footprint with all the basics you need before you build out a webserver for example. Give me SSH + Yum and Vim and we're good to go ((I'm sure I'm grossly misrepresenting what I need in a system, but you get the idea. Xfonts... WTF for? you know what I mean)).