Throbber

A throbber is an animated graphical control element used to show that a computer program is performing an action in the background (such as downloading content, conducting intensive calculations or communicating with an external device). In contrast to a progress bar, a throbber does not convey how much of the action has been completed.

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). Clicking on the throbber itself might perform some specific action (for example go to the program's website, pause or cancel the background action).

One of the early (if not the earliest) uses of a throbber occurred in the NCSA Mosaic web browser of the early 1990s, which featured an NCSA logo that animated while Mosaic downloaded a web page. As the user could still interact with the program, the 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/".

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