Flat Graphical User Interface Design is a design trend or philosophy which consists of using simple graphical elements without embellishment and bells and whistles for designing digital user interfaces. The design process focuses on colors, typographies and content. Flat design is a minimalist approach and a search for simplicity.
Flat design has been boosted by mobile devices and touchscreens that require simplicity. The Windows 8 tiles are often cited as a symbol of flat GUI design. The growing adoption of responsive design is also a trend favoring flat design.
The design process is called flat because no superfluous effects are used to give relief to graphical elements.
Flat design is often opposed to skeuomorphism design.
The Windows 8 illustration of flat design: