Cloud-based virtual representation of an IoT device that stores its last known state and desired future state, enabling applications to interact with devices even when they're offline. Like leaving a note for someone who's away, so they know what to do when they return.
A smart home app updates a thermostat's device shadow to set temperature to 72°F. When the device comes online after a network outage, it reads the shadow and applies the setting.
AWS IoT Device Shadow and Azure IoT Hub Device Twins both store a device’s reported (last known) state and desired state so apps can act even when devices are offline. Google Cloud and OCI don’t have a single, directly equivalent first-party feature with the same “shadow/twin” semantics; similar patterns are typically built using their IoT/streaming + database components or partner solutions.