1. <jargon> A placeholder term used to stand for a real object in didactic examples (especially database tutorials). Legend has it that the original widgets were holders for buggy whips. "But suppose the parts list for a widget has 52 entries..."
2. In a graphical user interface, a combination of a graphic symbol and some program code to perform a specific function. E.g. a scroll-bar or button. [possibly evoking "window gadget"] Windowing systems usually provide widget libraries containing commonly used widgets drawn in a certain style and with consistent behaviour.
In Microsoft Windows GUI programming, these are generally known as "controls".