A device instance is one particular occurrence of a device as Windows tracks it, combining the device's identity with where and how it is connected. Each instance gets its own entry, settings and driver assignment.
In practice
Plug the same USB flash drive into two different ports and Device Manager, with hidden devices shown, may list two entries. Each is a separate instance, identified by a device instance path that includes the bus, identifiers and a connection-specific portion.
Common confusion
An instance is not the same as a model. Two identical mice are two instances of one model. Nor is it the same as a driver; many instances can share one installed driver. Instance paths can contain serial-derived parts, so they are best kept out of public posts.
Why it matters to readers
Understanding instances explains duplicate entries, remembered devices and why settings sometimes appear to reset when you change ports. It also clarifies event histories, which are kept per instance rather than per model.
A closer look
Windows stores settings per instance, which is why a webcam's chosen resolution or a network adapter's advanced options may not follow the device to another port. Some devices report a unique serial number, letting Windows recognise them on any port and avoid creating new instances; others do not, so Windows relies on location instead. Both behaviours are normal.
