A small self-contained interface part doing one job. Named vaguely on purpose, since the point is that it is interchangeable.
First recorded in the early 20th century, of unknown origin, perhaps a fanciful alteration of gadget, named deliberately vaguely since the whole point of a widget is that it can be swapped for another.
A small self-contained interface part doing exactly one job, named vaguely on purpose because the whole point is that it can be swapped.