Dry means that all of the fermentable sugar has been consumed by the yeast. That's all.
Now, a byproduct of all of the fermentable sugar being consumed happens to be a lack of sweetness. Here's another way to look at it. I make two ciders, they both have the same level of sweetness. One had the fermentation processes halted leaving remaining fermentable sugars behind, thus "naturally" sweetening the cider. The second had all fermentable sugars fermented by the yeast and was then back sweetened. Despite having the same amount of sweet, the second is dry, the first is not. You could back sweeten a dry cider so sweet that it's undrinkable, but it's STILL a dry cider because all of the fermentable sugar was fermented. The opposite of "Sweet" is "Bitter", not "Dry".
At least that's how a few cider makers in Normandy explained it to me.
<ducks>