adjective – level and smooth, with no curved, high, or hollow parts
Much of the countryside in that region is very flat.
Although the word 'flat' as a noun means a series of rooms to live in, which is part of a larger building, its adjective form gives us a clue about the place of this word in design.
The flat design term is one of the a modern design styles of user interface design. The reason behind the association of this style with the word 'flat' is that mostly 2d illustration elements are preferred in visualizations. As you may realize when you look over flat design examples, it is more common to use bright, high-contrast colors and bold typography instead of gradients, textures, third dimension.
Futura PT Condensed Bold is available on Adobe Fonts.