What is Third-party Doctrine?

Legal Definition
The third-party doctrine is a United States legal theory that holds that people who voluntarily give information to third parties—such as banks, phone companies, internet service providers (ISPs), and e-mail servers—have "no reasonable expectation of privacy." A lack of privacy protection allows the United States government to obtain information from third parties without a legal warrant and without otherwise complying with the Fourth Amendment prohibition against search and seizure without probable cause and a judicial search warrant. Libertarians typically call this government activity unjustified spying and a violation of individual and privacy rights.
-- Wikipedia