flatten layers - technical definition

To bring all levels of a multi-layered image down to one plane. High-end graphics programs such as Photoshop provide a native file format that supports multiple layers. It enables image elements to be placed into different layers so they can be moved independently of each other. In order to save the layered image in a common format such as TIFF, BMP or GIF, which support only one layer, the image is said to be "flattened."