A simple definition of a social intranet is an intranet that has been extended (and integrated) with social tools. This definition is OK, but it's pretty tool-focused. To me a social intranet is an intranet that is focused on and supports people. More specifically, it supports people and their networks so that they can do their work (or do business).
In the next post I'll write about the first step towards a social intranet. In the meantime, what is your definition of a social intranet?