I feel hopeless asking this. The word in question as I understand it describes that a theory has to account for all observables under discussion without exception.
In historical linguistics it is based on regular sound-laws and to a lesser degree on common sense. There's no special name for that but the Neogrammarian school of thought has been attached as a symbol. Some have used the more generic term which is in question without explicitly referencing Neogrammarians.
The last time I checked, long after I read it once in a paper, I have found only Geology related usage and shrugged it off. Some time later it occured again in a more familiar context on Wiktionary, so I have reason to believe that it actually has currency. Now, however, I remember it as "Unitarian" which brings up denominations of churches, pretty much exclusively, so I'm afraid I misremember. It very likely starts with un(i) is all I can say.
In reality I am looking for a critique of this principle as applied in practice.