To combine together multiple layers and other elements of a digitally manipulated or composite image into one. Usually final step of working with layers prior to saving images in standard image format. Otherwise, save must be in native format.
To permanently combine selected objects in a PhotoDraw picture into a picture made of a single object. After saving a picture in a flat format, you can no longer edit its individual objects.
To convert multi-layered files to single layer files. This greatly reduces file size. However, once flattened, the layers of the file cannot be edited.
To remove structural information, esp. to filter something with an implicit hierarchical structure into a simple sequence; also tends to imply mapping to flat-ASCII.