Hi Birdy
I don't think you need 'vocabulary'. When I'm asked 'what is it?', I just say it's a form of hands-on healing that's passed on from teacher to practitioner, and which channels healing energy to the parts where it's needed.
What often amazes me is - the number of people who call me to ask for a reiki treatment who, when I ask if they know what it is or what to expect, say they have no idea!!! :-)