An internet accessible device is any system exposed to the public internet and reachable from outside the local network. That includes servers, routers, industrial controllers, cameras, and remote management interfaces. Exposure alone is not a breach, but it expands the attack surface because anyone on the internet can find and probe the device.
In real attacks, adversaries scan for these devices, test weak passwords, hunt for unpatched services, or abuse exposed admin portals and industrial protocols. In defense, the goal is to reduce unnecessary exposure: place remote access behind a gateway or VPN, segment networks, inventory public-facing assets, and monitor for unusual access. In OT environments, a single exposed controller can become an entry point to more sensitive systems.