OK, I assumed most people on the English-speaking Internet were aware of the furry fandom. Even if through a somewhat negative lens, most of the time.
But yeah, "a furry" refers to either an animal character anthropomorphised to a large degree, a fan of fiction or art featuring such characters, or to a person with an alter ego (often used when interacting online) that's an anthropomorphic animal.
This is interesting to me. Would that make any massive fan of donald duck/mickey mouse etc. a furry? It doesn't matter anyway, if people feel they are furries, they are furries. Never understood the people who get weirded out by the term, unless they have completely misguided notions on what it means.
I think there's a social component of being a furry. Kinda like "trekkie" implies being part of a certain fan community in addition to enjoying Star Trek.
In general, I think furries are vaguely defined for a very "I'll know one when I see one" reason. You know fursonas, the fictional furry alter egos people have that I mentioned earlier? I've known a guy whose fursona was a train. A train with no humanlike physical characteristics, mind you. I've met more than one person whose fursona was a human. Sure, those "fursonas" are in part at least tongue-in-cheek, but they exist. On their face, they make no sense and don't qualify as "furry". But they're accepted as fursonas because their "owners" are part of the furry fandom.
And there are plenty of people whose fursonas are fictional animals, animal hybrids, griffins, dragons and other mythological creatures. They are not technically anthro animals, but nobody really cares that much. EDIT: and taurs are, like, the most furry thing ever, and they're clearly not even close to actual animals. A taur is like a centaur, except the lower half is any four-legged animal (like a lion, for instance), and the upper half is a furry character instead of a human. I'm pretty sure I've never seen a taur character outside furry fandom circles.
On the other hand, when making fun of edgelord Batman fans who insist Batman should be serious and dark and brooding, the joke is made quite often that "Batman" is Bruce Wayne's fursona. And while I've seen - again, tongue-in-cheek - elaborate justifications of this position, and it *technically* fits, very few people would make a serious claim that Batman is actually a furry in any meaningful sense. Why? Because of social significance and intention.
Reminds me of a quote of what Salvador Dali allegedly said: "The only difference between me and a madman is that I am not one.". Being a furry is kind of an inverse of that. The only difference between me and a non-furry is that I *am* one.
So - is a huge fan of Mickey Mouse a furry? Not automatically. *can* a huge fan of MM be considered a furry based on liking MM a lot, and nothing else? Sure, why not. I think the general attitude among furries is "everyone's invited* ", and I think that impacts the definition being very loose and broad.
*Except Nazis. Nazi furs, famously, can f*ck off.