The DEFINED attribute indicates that PL/I is not to allocate storage for the variable, but is to map the description of the variable onto the storage of another variable called the base variable. The DEFINED attribute provides a way to access the same data using different names .
DECLARE NAMES(10) CHARACTER(5) DEFINED (LIST),
In this example, the variable NAMES is a defined variable; its data description is mapped to the storage occupied by the variable LIST. Any reference to NAMES or to LIST is resolved to the same location in memory.
Please correct me if i am wrong.