An external service is a system outside the local device or network that provides part of a feature, such as content delivery, authentication, search, or media processing. The local application may look complete, but it actually depends on a remote provider to make the feature work.
In cyber security, external services matter because they expand the trust boundary. If the service fails, is discontinued, or is tampered with, users may see outages, broken workflows, or unexpected behavior. Attackers can target these dependencies through service abuse, supply-chain compromise, or DNS and routing attacks. Defenders track external services to assess availability risk, set fallback behavior, reduce exposed dependencies, and decide which remote features are acceptable on managed systems.