There must be dozens of names for the Flat Cap so which one is your usual one?
Er, what’s wrong with ‘flat cap’?
I can’t off hand think of any other words for it.
My gf brought me one back from Ireland. I look great in it. I call it my Irish cap.
Flat cap.
Unless it has a pom-pom on the top. Then it is a tam.
That’s usually a “newsboy”; occasionally it’s a “golf cap.”
Salt and Pepper Cap, though that really refers to ones with a particular fabric pattern, it’s the name in my head for the style in general.
Neat you should ask, because it was only the other day that I first saw Flat Cap referred to as such. Of the names in that article, I think I would have used driving cap or Newsboy cap to distinguish the style, although I had always heard snap-brim before that other day.
Since I educated myself on the topic, I wondered how many others have their own names for such headgear.
For my own usage, a tam without a pom-pom is a beret. No brim.
I always knew it as a driving cap.
I’ve always known it as a newsboy. I knew instinctively which style you meant with the term “flat cap” though, without ever having heard it used.
I like the Russian term for them - aerodrome [кепка аэродром]. Stereotypically worn by Georgians or Azerbaijanis.
Damn if there aren’t some extreme examples there! 
Flat cap is a generic term. It could be a newsboy cap, a driving cap (golf cap), a floppy baseball cap, or one of those new kinds that are like a baseball cap but the cap part is connected to the brim to make that duckbill look.
Scally cap.
My father called them mugger caps. My wife bought me one a few years ago, I referred to it as a mugger cap, and got a weird look. I looked up hats on Wikipedia and found them referred to as flat caps, and that is how I refer to them now.
Driving cap for silly gits.
Kangol
I would have called it a newsboy or driving cap.
In the 70’s I worked with a middle-aged Irishman who had emigrated to the US in his 20’s. He sometimes wore one of those hats, and told me they were called “caps.”
Bocher hittel, or boys’ everyday hat, in Yiddish. A lot of boys wore those kind of hats at the Jewish schools, but had nicer hats for Shabbes, and then on their Bar Mitzvah, they’d get a wide-brimmed, sort of fedora-type hat like the men wore, and it was a big deal to have it, because it showed everyone you were a Bar Mitzvah.
Some adult men wore Bocher hittles as everyday hats, but they were usually men in their late 20s or early 30s. Young enough not to feel silly in one, but old enough that they didn’t feel the need to show off their Bar Mitzvah hat.