Magic eraser tool
From The Glossary of Digital Photography
A tool in Photoshop designed to erase a color specified by the user. The user can specify how close to that color the tool should work. For example, a blue sky is not all one shade of blue. By giving the tool a wider tolerance, the user can erase the sky without touching the foreground, as long as the foreground is not blue. Other settings adjust this tool to behave in different ways. This tool is very useful for removing objects from their background by erasing the background. After it erases, the erased area is transparent.
