An API is a defined interface that lets one software system request data or trigger functions in another system in a controlled way. APIs are common in web apps, cloud services, industrial platforms, and OT integrations because they make systems easier to connect without exposing internal code or databases directly.
In cyber security, APIs matter because they often become the boundary where authentication, authorization, logging, and rate limits are enforced. If those controls are weak, attackers may abuse an API to steal data, change settings, or pivot deeper into a network. Defenders treat APIs as high-value assets: they inventory endpoints, require strong identity checks, apply least privilege, validate inputs, monitor for unusual calls, and revoke access quickly. In industrial environments, APIs also help expose machine data for compliance or interoperability, but they must be segmented so convenience does not create new attack surface.