SCN

System Change Number (SCN) This is a sequential counter, identifying precisely a moment in the database.
This is the most accurate way to identify a moment in time.
SQL> SELECT CURRENT_SCN FROM V$DATABASE; -- to get the current change number.

CURRENT_SCN
-----------
  300121782

OR
SQL> SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL;
GET_SYSTEM_CHANGE_NUMBER
------------------------
               300121802

ORA_ROWSCN: a pseudocolumn of any table that is not fixed or external. It represents the SCN of the most recent change to a given row, that is, the latest COMMIT operation for the row. 

SQL> SELECT ora_rowscn, ename, sal FROM scott.emp
  2     WHERE empno = 7788;
ORA_ROWSCN ENAME   SAL
---------- ---------- -------
 299751149 SCOTT          3000   --- The latest COMMIT operation for the row took place at approximately 299751149

You can use function SCN_TO_TIMESTAMP to convert an SCN, like ORA_ROWSCN, to the corresponding TIMESTAMP value. 

SQL> SELECT ora_rowscn, ename, sal,SCN_TO_TIMESTAMP(ora_rowscn)
  2     FROM scott.emp
  3     WHERE empno = 7788;

ORA_ROWSCN ENAME             SAL SCN_TO_TIMESTAMP(ORA_ROWSCN)
---------- ---------- ---------- ------------------------------------
 299751149 SCOTT            3000 05-DEC-10 01.19.43.000000000 PM

 SQL> select TIMESTAMP_TO_SCN(to_date('12/5/2010 1:19:43 PM','mm/dd/yyyy HH:MI:SSPM')) SCN from  dual; -- timestamp to SCN, It may have a precision of +/- 3 seconds

       SCN
----------
 299751145