Illumos is the kernel, and SmartOS and OpenIndiana are both Illumos distributions.
The OS is packaged the same way Linux is, with different distributions maintained by people outside the kernel devs.
The OS is packaged the same way Linux is, with different distributions maintained by people outside the kernel devs.
I think it's not ideal to use the term "distribution" for their downstreams like SmartOS etc. because it reminds people of all these GNU/Linux distributions, but a better term doesn't immediately come to mind...