PRISM was a global military organisation that replaced the United Nations Intelligence Taskforce. It was created shortly before 2066, at a time of global stability, wealth, and confidence - this showed in its flying HQ SKYHOME, a "white elephant" secret base that gave itself away with its contrails until PRISM techs could figure out a solution.
It fought the Myloki from 2066-68. The Myloki could possess or replicate people and used this to fight a campaign of sabotage, which PRISM countered in secret. While the organisation won, the UN banking system collapsed as PRISM had needed to blow its budget.
In 2070, the organisation was exposed in the memoir Message Is Clear and was forced to shut down. Commander Hal Bishop took the remnants of it underground as SILOET, using what remained of PRISM's resources. (PROSE: The Indestructible Man)