TRANSACTION_ID

You can get the TRANSACTION_ID of a transaction using the DBMS_TRANSACTION.LOCAL_TRANSACTION_ID function.

SET SERVEROUTPUT ON
DECLARE
    l_transaction_id VARCHAR2(64);
BEGIN
    l_transaction_id := DBMS_TRANSACTION.LOCAL_TRANSACTION_ID;
    DBMS_OUTPUT.PUT_LINE(l_transaction_id);
END;

This is extremely useful when used in conjunction with the Invisible Columns feature to track the changes made by a transaction to a table.