A component that, by default, displays an integer value within a bounded interval. A progress bar typically communicates the progress of some work by displaying its percentage of completion and possibly a textual display of this percentage.
To indicate that a task of unknown length is executing, you can put a progress bar into indeterminate mode. While the bar is in indeterminate mode, it animates constantly to show that work is occurring. As soon as you can determine the task's length and amount of progress, you should update the progress bar's value and switch it back to determinate mode.