A doughnut is a fried piece of dough that is usually covered or filled with something sweet.
Herbert Rossoff ate doughnuts and often complained to Douglas Pabst about the standard of the doughnuts provided to the staff at Incredible Tales, saying they were stale. Pabst disagreed, but agreed to provide Rossoff with fresher doughnuts the following day after Rossoff threatened to go and work for Galaxy. (DS9 episode: "Far Beyond the Stars")
However, everyone in the Incredible Tales office knew that the unsold doughnuts they saw in a bakery at the end of the day would be in their office the next day or the day after. (DS9 novelization: Far Beyond the Stars)