Hi Kristian:
I don't believe the PMBOK guide refers to it either. In these situations, I often visit my favourite PMP websites and try to find an explanation that I can best digest and document it in my notes. I sometimes start at Wikipedia (knowing that it may not be 100% accurate) but then follow the references sited. Youtube has been surprisingly full of useful content. There isn't one true source with all the answers.
Tracey