The proper term is Autumn, but I usually say Fall.
At school I was supposed to call it 'hydref' (which coincidentally is also the word for October). I don't think I've ever called it fall in my life.
That time between holy tit hot and holy smurf cold.
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I say Fall, and I'm actually really surprised that most people here seem to say Fall as well. I figured it was more acceptable to say Autumn.
If I had four daughters, I'd name them...
I use both interchangeably, but I probably use "fall" most often.