The context of the conversation narrows the meaning of "fern" down, usually anyway. If you are talking vintage Gibson F-5s, it will be as John stated above. 40 years ago this was about the only meaning.
In the 1970s other manufacturers like "Kentucky" and other brands started using fern inlay patterns. Then Gibson after retooling in the late 1990s as stated above designated some of their mandolins' model numbers as "Fern", like my Gibson 2002 "fern" model (on the label) . I think Gibson started this followed by Weber. Then a few others maybe. But then the brand "The Loar" showed up. throwing more confusion into discussions. Add to all this Gibson's "Master Model" model designation of the master model line of their mandolins. Along with all the other manufacturers "master model" offerings. So that, you can have a Gibson brand, master model "fern" and even a Gibson model no. "Master Model Fern" which may look almost just alike but be constructed very differently. So, that clears it up, right?