illumos is a fully open community project to develop a reliable and scalable operating system. It began as a fork of the former OpenSolaris operating system. The illumos code base forms the foundation of distributions including a storage appliance from Nexenta, a cloud platform from Joyent, and the general-purpose OpenIndiana.
illumos is pronounced i-llu-MOS and written in lowercase. The name 'illumos' ties in with Sun and light.