flat - apartment
In British English, a flat is a set of rooms for living in, usually on one floor of a large building.
She lived in a tiny furnished flat near Sloane Square.
In American English, a set of rooms like this is usually called an apartment.
It is a six-story building with 20 luxury two- and three-bedroom apartments.
‘block of flats’
In British English, a large building containing flats is usually called a block of flats.
The building was pulled down to make way for a block of flats.
‘apartment block’ and ‘apartment building’
In American English, and sometimes in British English, it is called an apartment building or an apartment block.
He lives on the ninth story of an apartment block on Charlesgate East.
Several apartment buildings were destroyed in the fire.