XKeyscore (abbreviated as XKS) is a formerly secret computer system first used by the NSA for searching and analyzing internet data it collects worldwide every day. Edward Snowden and Glenn Greenwald explained XKeyscore as being a system which enables almost unlimited surveillance of anyone anywhere in the world, while NSA said that usage of the system is limited and restricted.
XKeyscore consists of over 700 servers at approximately 150 sites where the NSA collects data by extracting certain types of metadata (like phone numbers, email addresses, logins, and user activity) and index them in metadata tables, which can be queried by analysts. XKeyscore has been integrated with MARINA, NSA's database and analysis toolset for intercepted internet metadata and stores it for a year.