An attack using traffic from multiple sources to exhaust resources and disrupt legitimate access. Defence may require upstream filtering and distributed infrastructure. An application firewall alone cannot handle every kind of DDoS.
A community-developed operating-system distribution. It provides packages and updates; release-branch choice affects stability, available versions and support.
A plan for restoring services after major disruption. It defines roles, dependencies, recovery order and timing goals; it must be exercised, not just documented.
A hierarchical, distributed naming system. DNS records can identify IP addresses, mail servers and other information. A domain need not correspond to one website or server; DNS answers are also cached.
A platform for building and running application containers. Containers usually share the host kernel; they are not automatically full virtual machines or absolute isolation boundaries.