I've searched through the archives, but can't find a description. What does it mean when a bike has "butted tubing."

I don't mean double-butted, triple-butted, or quad-butted. I know what those mean.

I mean "butted tubing." Is that the same thing as single-butted tubing? Or is it something else? I know I read it here once, but I just can't find the answer.