What constitutes a Pale Ale? I ask this because I just ordered an "American Pale Ale" and it's actually quite dark in color. So how can I tell a pale ale?  What actually defines an ale as pale?
It's historical.

The "Pale" in Pale Ale is mostly historical. In the 18th century, most beers were dark due to being produced with barley malt that was kilned or roasted over wood fires. But from England emerged a new technique using pale malts, cured in coke-fuelled kilns. This applied to both ales and lagers, resulting in beers that were bronze, copper or gold—and while you wouldn't look at such colors today and consider them notably "pale," they certainly were pale in contrast to beers back then. In the turn of the 19th century and beyond, the style spread from England to the rest of Europe—


  When a new brewery was built in Pilsen in Bohemia in 1842, a coke-fired kiln was imported from England. The result was the first golden lager, Pilsner Urquell, which was made possible by British ingenuity and technical advance.


http://www.beer-pages.com/protz/features/ipa.htm

Note that American Pales are distinct from India Pales as well as from American India Pales. See my answer about the differences between styles of IPAs.