this contains the commited and uncommited data stored in
rollback table space prior to 9i. in 9i, it is called as
undo table space.

ROLLBACK SEGMENT OR UNDO SEGMENT IS THE AREA IN UNDO TABLE 
SPACE WHERE CHANGED OR NEW UNCOMMITTED DATA RESIDES.

WHEN WE COMMIT, UNDO SEGMENT DATA BECOME UNUSEABLE. BUT IT 
STILL RESIDES IN THE UNDO SEGMENT FOR 600 SECONDS.

TO REMOVE THE CONSITANCY ISSUE FROM ORACLE DB.

WHEN WE COMMIT, UNDO SEGMENT DATA BECOME UNUSEABLE. BUT IT 
STILL RESIDES IN THE UNDO SEGMENT FOR 900 SECONDS.

it is called undo retention period