Most of the time, in my experience, the bottom floor would be the "ground floor" and the floor above it would be the "first floor." But, supposedly, in the US, the first floor is the same as the "lobby" or "ground floor."
Where I grew up, the first floor was the floor above the lobby (like in the UK).
There are times when the "ground floor" is the "first floor," though.
The level of the "ground floor" is, many times, known as the "lobby."