I think coach or facilitator also applies, but it's all a balance. Sometimes our kids need us to be the voice in charge, the expert on the content or the process (or both), sometimes we need to step back a little, and sometimes we need to step back a lot. It's not a binary proposition. :-) As one who prefers to step back, I have to remind myself that there are learners who don't feel like they've really "learned" unless they hear the expert voice. That means I have to plan specifically to lecture sometimes, though I'm careful to do it after students have already completed their PBL units. I consider it a part of the debriefing and reflection process.