One of my favourite aspects of Less Wrong is that we've created a community where it is easier for us to communicate with each other because we have a shared understanding of certain concepts. A large amount of highly quality content has been posted to the site since it started, so I think it would be a useful exercise to reflect on which ideas should become part of this common knowledge for this edition of Monthly Meta.
I'd suggest that this should work as follows: each post should contain one concept, with a short definition and an explanation of why you think it is important. Please don't submit your own ideas. I'll go first.