PRAGMA EXCEPTION_INIT

To handle error condition that dont have predefined name, you must use OTHERS handler or Pragma EXECEPTION_INIT. A pragma is compiler directive that is process at compile time, not run time

In PL/SQL pragma EXCEPTION_INIT is declared in DECLARE section and it associate with oracle error number

SET SERVEROUTPUT ON
DECLARE
  invalid_depto EXCEPTION;
  --check constraint violated
  PRAGMA EXCEPTION_INIT (invalid_depto, -2290);
BEGIN
 INSERT INTO EMP (empno, ename,salary,deptno)
  VALUES (90,''Justin',2000,'X');
 COMMIT;
EXCEPTION
 WHEN invalid_depto THEN
  DBMS_OUTPUT.PUT_LINE('Dept No Check constraint violated');
END;