All rights reserved. This is because when people use the term, most of the time there is no way to know what they intend to mean by it.
Some uses overlap others, some have little or nothing to do with other uses.
And rarely is it clear from context which meaning is intended. Not in the SCA, where the presumed period is about 500-1600 unless explicitly indicated otherwise. It is a precise technical term, and can be freely and unambiguously used in contexts where everyone understands that technical term. But whenever there is someone involved in the discussion who may not know the technical terminology, it should be defined to the extent necessary. In the Middle Ages and Renaissance, people who spoke one of the languages from the Celtic language family didn't consider themselves to have any special connection either with others who spoke a different Celtic language or with the ancient continental Celts of the time of the ancient Greeks and Romans.