NISA stands for the News International Staff Association and is the trades union that represents staff employed by the News International organisation in the United Kingdom.
It is one of the few unions that has been refused a certificate of independence by the certification officer on the grounds that the employer had "played a close and controlling role in the development of the organisation" and that NISA still had "some way to go before it could meet the statutory definition of independence by removing itself from being liable to interference by the employer". (That's Rupert Murdoch for you.)