The word plainchant, according to the Oxford English Dictionary, was originally a synonym for Gregorian chant, but that's because the originators of the term probably knew of no other chant dialects.
To me it doesn't really matter what the terms once meant. We know better now, and it makes sense to use the term "plainchant" to describe the dialects in general. Are we going to call David Hiley and tell him that his book has a bogus title? He's a very nice man.
But since you're trying to clear this up by the end of the week, I guess wikipedia would be ok. Just don't tell my graduate students.
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