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bookmarklet

n. (context computing Internet English) A small piece of JavaScript code stored as a URL within a bookmark.

Wikipedia
Bookmarklet

A bookmarklet is a bookmark stored in a web browser that contains JavaScript commands that add new features to the browser. Bookmarklets are unobtrusive JavaScripts stored as the URL of a bookmark in a web browser or as a hyperlink on a web page. Bookmarklets are usually JavaScript programs. Regardless of whether bookmarklet utilities are stored as bookmarks or hyperlinks, they add one-click functions to a browser or web page. When clicked, a bookmarklet performs one of a wide variety of operations, such as running a search query or extracting data from a table. For example, clicking on a bookmarklet after selecting text on a webpage could run an Internet search on the selected text and display a search engine results page.