Definition of Franking privilege
The privilege of sending certain matter through the public mails without payment of postage, in pursuance of a personal or official privilege. The privilege granted to members of Congress to send out a certain amount of mail under signature without charge. See 39 U.S.C.A. § 3210 et seq.
That's the definition of Franking privilege in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.