http://dictionary.reference.com/browse/autumn
au·tumn
   [aw-tuhm]
noun
1. the season between summer and winter; fall. In the Northern Hemisphere it is from the September equinox to the December solstice; in the Southern Hemisphere it is from the March equinox to the June solstice.

Agree--remove (Brit.). Widespread in the US.

Zumal es zu "fall" kein Adjektiv gibt, zu "autumn" aber mit "autumnal" eines meiner Lieblingswörter in der englischen Sprache.

I agree that the [Brit.] marking is wrong and should be removed. I could have sworn this had come up and been resolved long ago. All I found was this one thread in the archive, but it's at least pretty extensive, so it might even be worth linking to the dictionary entry.

While 'autumn leaves' or 'fall color' are probably more common collocations, there's nothing wrong with 'fall leaves,' as Amy-MiMi pointed out in the other discussion.

If the definition in #1 is from a reputable published dictionary, perhaps American Heritage, it would be good to cite that actual source, since the internet site dictionary.reference.com is not itself as reliable. (As I remember, it was one of the worst offenders with regard to privacy and cookies.)