Most of us can take one look at something ad say if it's a dragon or not with relative ease. So far the only features that all dragonic creature have in common is they're not real. they capture the imagination, and that they share physical traits of multiple creatures. So is identifying a dragon like identifying an illness where there is a list of dragony traits and if a minimum number of them show up in any configuration it's a dragon? Or are there just features that exclude it from being a dragon? So far I've seen them depicted in actual myths and legends as hyper evolved reptiles and dinosaurs, winged reptiles of any type, a serpent-like creature with a twist, even giant snails, and black fey wolves with curly hair, and many of these with little to no intelligence. I would really appreciate some perspective on this. I want my definitions to be correct compared to actual mythological references so that the world in my books feels more like a real legendary tale and not some collection of reimagined pop-art monsters. Quite frankly If I can't wrap my head around what a dragon is suppose to be then what right do I have to tell my readers? I'm not going to be one of those people that slaps a duckbill and a pair of macaw wings on a fox and pass it off as a true gryphon, and I definitely won't apply that ridiculousness to dragons and still call it a dragon.
Dragons are anything that meet a handful of specifications in a set of criteria that keeps getting increasingly larger, or anything which is based off of something which is once said to be a dragon
Let's go through a few character traits of dragons. Dragons are large, except when they aren't. Dragons are intelligent, except when they aren't. Dragons breathe fire, except when they breathe ice, except when they breath acid/poison/air/(various other elements), except when they don't. Dragons are evil, except when they aren't. Dragons can fly, except when they can't. Dragons are serpentine, except when they're lizardlike, except when they've decided to look and function like humans. Dragons have scales, except when they don't.
The reason for this is simple - there's not one origin for the concept of a dragon. When you boil it down, dragon are featured in dozens of cultures around the globe, except they aren't. Take a look at the list here. You'll find dozens of 'dragons' mentioned from every mythology, ranging from Bahamut, Tiamat, Naga, Orochi, wyverns, wyrms, Quetzcoatl, etc. And they have nothing in common, save for this term 'dragon' that links them all.
Mythology across cultures is weird, because mythology in different cultures is usually different. When two cultures meet, they will identify various beliefs or myths with myth of beliefs that each respective culture had. Europe, for instance, had this catch-all term that they used massive reptilian beasts: 'dragons'. When they traded with China, they ran across beasts that China believed in - the Lung. For ease of understanding, and also because cultural misconception was basically a sport in the Middle Ages, the Europeans just chose to translate it as dragon, despite the fact that the two sets of creatures really didn't have that much in common with each other. And the cycle just kept on repeating - every time a new culture was incorporated, they just took the closest thing said culture had to a dragon and then proclaimed 'well, this is the Native American/Axtec/Japanese/etc. version of the dragon'.
Fast forward to modern day (well, a good deal of this culture osmosis and mis-identification happened near the modern day, within the last few hundred years) and basically we have dragons as this catch-all term that can refer to dozens, if not hundreds, of really distinct creatures because of sloppy translating and because humans like identifying new things with familiar terms. Not to mention that the latest fad in fantasy writing is 'reinterpretation' (I'm not saying it's a bad thing, but it is a thing that happens), so a lot of authors who write fantasy are pulling off twists on dragons and giving them new features.
So, to answer your question, dragons are anything that meets the vast historical or cultural standards for a dragon, or else whatever the author feels like pointing at and saying 'See this? This is now a dragon for no other reason than I can say it is and no one will fight me over it.' I know this probably isn't the answer you're looking for, but unfortunately, it's the truth.
They are a metaphor for the unknown and dangerous wilderness.
In the modern mind, images of dragons on the map marked what was past the edge. A dragon is a metaphor for the perilous unknown. I am proud of the Wikipedia author who thought to quote Plutarch.
I might very well say of those that are farther off, beyond this there is nothing but prodigies and fictions, the only inhabitants are the poets and inventors of fables; there is no credit, or certainty any farther.
Plutarch, quoted in Wikipedia: https://en.wikipedia.org/wiki/Here_be_dragons
Defining characteristics: the dragon must be prodigious, possibly mythical. Fabulous and dangerous. Exploring and conquering the wilds could bring fame and fortune; so too conquering a dragon. Many people who ventured into the unknown hoping for fame and fortune never returned. Dragons will keep you.
I was delighted by the close of this article on "Here Be Dragons". https://www.gislounge.com/here-be-dragons/
The term (Here be Dragons) has also been adopted by programmers to comment on code that is unintelligible but still works. The phrase is intended to serve as a warning for other programmers to not tweak the code in fears of breaking it.
Leave dragons alone!