To make something flat and smooth by pressing it down or rolling it, or to compress something so it takes up less space.
From Old Norse 'flatr,' meaning 'flat.' The '-en' suffix is a common Germanic ending that turns adjectives into verbs, so 'flatten' literally means 'to make flat.'
Flattening is used in art, cooking, metalworking, and even image compression—in computer graphics, 'flattening' an image combines all the layers into one, which makes the file smaller! The Great Plains of North America were actually flattened by ancient glaciers.
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