I've been hearing it both ways my entire life, but for the last several years I've been hearing the word autumn less and less.
I just did a quick google search and it said that fall is primarily used in North America to describe the third season of the year.
It also says that autumn is a more formal way to say fall.
Have we gone away from formal language? There are so many slang and abbreviated words in our daily conversations so I guess it makes sense that we call it fall.
What do you call this season? Fall or Autumn?