Write what you know has always be subject to misinterpretation, or literal translation.  However, one of the best definitions came from Allen Wold at a science fiction convention (paraphrased).  It's everything that you know, everything you have learned from other people, and everything you can learn from research.  But I'd also add a caveat: It also needs to be something that you can reliably write.  I'm not a doctor, but I could probably research medical science -- but I'd have to do so much research just to be credible that it wouldn't be worth my time.

Linda Adams - Soldier, Storyteller http://garridon.wordpress.com/