A throbber is a graphic found in a graphical user interface of a computer program (especially a web browser) that animates to show the user that the program is performing an action (such as downloading a web page). Usually the throbber is found on the right side of a program's toolbar or menu bar. The form the throbber takes varies, but it is common for it to be the logo of the program it is part of.

Most of the time the throbber is a still image (known as its resting frame), but when the program is performing an action the throbber begins to animate in a loop to let the user know that the program is busy and has not frozen. Once the action is complete, the throbber returns to its resting frame. Normally, it is possible for the user to continue interacting with the program while the throbber is animating (one such possibility may be to press a stop button to cancel the action that the program is doing).

Often, clicking on the throbber itself will go to the program's website. One of the earliest (if not the earliest) uses of a throbber was in the NCSA Mosaic web browser of the early 1990s, which featured an NCSA logo that animated when Mosaic was downloading a web page. As the user could still interact with the program, the mouse pointer remained normal (and not a busy symbol, such as an hourglass); therefore, the throbber provided a visual indication that the program was performing an action.

Clicking on the throbber would stop the page loading; later web browsers added a separate Stop button for this purpose. An Easter egg was implemented that replaces the throbber with an image of the rotating head of Tom Magliery when browsing his home page. This Easter egg appears with any web site whose URL contains the substring "~mag/".

Netscape, which soon overtook Mosaic as the market-leading web browser, also featured a throbber. In version 1.0 of Netscape, this took the form of a big blue "N" (Netscape's logo at the time). The animation depicted the "N" expanding inward and then outward, thus explaining why these animations became known as throbbers.

When Netscape unveiled its new logo (a different "N" on top of a hill), they held a competition to find an animation for it. The winning design (featuring the "N" in a meteor shower) became very well-known and almost became an unofficial symbol of the World Wide Web. Later, Internet Explorer's blue "e" enjoyed similar status, though it has since been replaced by the Microsoft Windows flag as the throbber in most versions of the browser, and again by a glowing blue circle in Internet Explorer 7.

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