What is a dragon? Eastern dragons
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What is a dragon?

Random House Webster's College Dictionary

drag-on (drag`en), n. (Gk, drakon, kind of serpent, probably orig. epithet, akin to derkesthai to look) 1. a mythical monster generally represented as a huge winged reptile with a crested head, often spouting fire.  2. Archaic. a huge serpent or snake.

Dragons are one of the most common myths. Nearly every culture has some type of monster or huge lizard that is capable of breathing fire or controlling weather. Dragons come in all shapes and sizes, colors and appearances, good and evil; how the dragon looks depends upon which culture is relating the story. They can be known by many names: wurm, wyrm, drake, lung, dragon, hydra.

Article: Dragons The Encyclopedia Mythica
Draconian.com: What is a dragon?