Pseudo-column

From Oracle FAQ

A pseudo-column is an Oracle assigned value (pseudo-field) used in the same context as an Oracle Database column, but not stored on disk. Examples of pseudo-columns are: SYSDATE, SYSTIMESTAMP, ROWID, ROWNUM, UID, LEVEL, CURRVAL, NEXTVAL, etc.

Examples

SQL> SELECT sysdate, systimestamp FROM dual;

SYSDATE   SYSTIMESTAMP
--------- ----------------------------------------
13-DEC-07 13-DEC-07 10.02.31.956842 AM +02:00

SQL> SELECT uid, user FROM dual;
       UID USER
---------- ------------------------------
         0 SYS

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