Hi,
any small device or appliance, especially one more ingenious than necessary
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A gadget is a whatsit, doohickey or thingamajig. Usually a smallish one.
It's basically a generic term for any useful item that people can never remember the real name of. It's usually a novel invention of some sort (or at least, it was when it was first produced).
These days most things we call "gadgets" are small electronics - phones are an example. But more mechanical items are also gadets - a wing corkscrew for example.