What makes a doughnut a doughnut?
Essentially, a doughnut is sweet, deep fried dough. Some purists also insist that they be round, with a hole in the middle, but, in my opinion, that would exclude any kind of filled donut. It can be topped, or filled with various things and most cultures around the world have some variety of pastry which could be called a donut. According to WiseGeek, "In the United States, a Captain Mason Gregory is credited with the invention of the classic round doughnut, which cooks quickly and evenly in a deep fryer."
Dough for a doughnut is generally one of two type, yeast or cake. Interestingly, although cake donuts are often referred to as old-fashioned, theories indicate it was probably the yeast donut that came first, using up left over dough scraps from other yeast containing pastries.
Donuts or Doughnuts?
The original spelling of the word is doughnuts and that is the prevalent spelling in most of the world. Like most things, however, the good old U.S. of A. has to go it's own way and spells it 'donuts'
For example, the Lihapiirakka is a dish found in Finland which is described as a 'meat donut' or meat pie.
My Malaysian friend at work says they make a curry puff with pastry dough that's kind of donut like.
I also found this recipe for a 'cronut' which is part croissant, part donut
Interestingly, these Cronuts are apparently a HUGE deal in New York City, as you can read here