Fourth Amendment: Part of the Bill of Rights that prohibits unreasonable searches and seizures and requires any warrant to be judicially sanctioned and supported by probable cause.
Patriot Act: An Act signed into law following 9/11 with intention to strengthen domestic security against terrorism; it expanded surveillance powers including wiretapping, business records access etc.
NSA (National Security Agency): A U.S. government agency responsible for signal intelligence and information assurance; often associated with global surveillance.