FCEB means Federal Civilian Executive Branch: the U.S. civilian federal agencies covered by certain CISA cybersecurity directives. It generally excludes the Department of Defense and other non-civilian branches, but includes many agencies that run public-facing services and sensitive internal systems.
In cyber security, the term matters because CISA can require FCEB agencies to take specific defensive actions, such as tracking known exploited vulnerabilities, patching on a short deadline, or reporting remediation status. Attackers often target these agencies because they offer high-value data, large attack surfaces, and long-lived legacy systems. For defenders, FCEB status defines who must comply, what timelines apply, and how quickly exposed systems must be found, prioritized, and fixed.