Here's the definition:
As I pointed out in the other thread, pull-ties seem to qualify as a safety device under (A).Title 18 S 921 (a)(34) The term ''secure gun storage or safety device'' means -
(A) a device that, when installed on a firearm, is designed to
prevent the firearm from being operated without first
deactivating the device;
(B) a device incorporated into the design of the firearm that
is designed to prevent the operation of the firearm by anyone not
having access to the device; or
(C) a safe, gun safe, gun case, lock box, or other device that
is designed to be or can be used to store a firearm and that is
designed to be unlocked only by means of a key, a combination, or
other similar means.