Is the bottom-most floor (on ground level not the basement) "ground floor" or "first floor" in America?
Normally there is a slight difference between the ground floor and the first floor depending on a commercial or residential building. Commercial structures refers to the entrance level as the first floor of the building and residential structures refers to the entrance level as the ground floor of the building and the first floor being above the entrance level.
In many buildings in New York City, ‘L’ (which stands for Lobby) is on the ground floor (I’m talking about elevators and apartment numbers) and ‘1’ is the 2nd floor (also talking about elevators and apartment numbers). My current Apartment number is 5G and I press 5 on the elevator to get to my floor but it’s actually what is called the ‘6th floor’. My last apartment had the same floor numbering system, too. So it’s not US vs UK thing. The level below ground floor is called ‘basement’ here by the way.
I've heard it called the first floor in areas where basements are uncommon. "Ground Floor" is used to describe the floor on the ground because the first floor is actually the basement. I have seen this practice used at universities. University elevators have a first floor, but the ground floor (usually floor 2) is denoted with a star to indicate the "Ground Floor".