Die Break "Cud"
The image to the left is an example of a 1989
Lincoln Cent that has a major die break, often
referred to as a "Cud".  

"Cuds" are a type of die break.  When an
area of the die breaks off, a gap in the die's
face is created.  The coins struck by these
dies display a raised blank section.  The
portion of the reverse of the coin opposite the
cud will be struck weakly due to the lack of
counter-pressure.   A die break can only be
called a "cud" if the break occurs near the rim
of the die.  Otherwise, they are just simply
referred to as a "die break".  

Sometimes the die break will not fall off
completely, and the resultant strikes are
referred to as "retained cuds".  

Images courtesy of Steven Bingham