An unpackaged, bare chip. A die is the formal term for the square of silicon containing an integrated circuit. Die is singular, and dice is plural. The terms die and chip are often used synonymously.
Dice on a Wafer
This picture of several dice on the wafer shows the various subsystems on each die (chip). This image is called a "beauty shot," because the different areas are colored for presentation. (Image courtesy of Texas Instruments, Inc.)