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The tertiary-source fallacy (TSF) is the idea that because something appears in a dictionary, encyclopedia, style guide, or other tertiary source that it must be correct (or, especially, that it is the only correct version or interpretation), and that it trumps other arguments and evidence. More specifically, it can be called the dictionary fallacy, encyclopedia fallacy, or style-guide fallacy.
Wikipedia treats tertiary sources like these as categorically not very reliable, and of highly variable quality. There are important reasons for this.
It is not fallacious to cite such a work in an article or offer it as evidence in a discussion. But advancing its view as if it ended the discussion, as if other facts and reason cannot surmount your pet source, is fallacious. Specifically, it is a form of the argument to authority fallacy.