(from Wikipedia entry)

A Web Widget is a portable chunk of code that can be installed and executed within any separate HTML-based web page by an end user without requiring additional compilation. They are akin to plugins or extensions in desktop applications. Other terms used to describe a Web Widget include Gadget, Badge, Module, Capsule, Snippet, Mini and Flake. Web Widgets often but not always use Adobe Flash or JavaScript programming languages.

(from Thomas Landsburg)

A Mobile Widget is generally speaking a small application, a little bit of code, that does one simple thing but does it well. The second important things about a widget, is that they can be combined together. The last one being the fact that usually, it’s fairly easy to develop a new widget (comparing to developing a full application).