Spinner

Indeterminate is the whole word. A spinner has no value, no denominator and no position, and that absence is not a gap in the component, it is what the component says: something is running, nobody knows for how long. Everything else follows from it. There is nothing to announce as a percentage, nothing to be wrong about, and no way for the reader to plan around it.

Which makes the first question the honest one: could this have had a number? If the work is bytes of a known total, steps of a known count, or seconds of a known duration, the reader deserves a progress bar or a progress ring, and a spinner in that slot is a determinate indicator someone gave up on. The two get confused constantly because a ring that spins and a ring that fills are the same drawing at a glance, so the tell is in the markup: a determinate control carries role="progressbar" with aria-valuenow, and a spinner carries neither. The second question is duration. A spinner is for a wait of a second or two; past that it stops reassuring and starts reading as a page that is stuck, and the right answer is usually a skeleton screen, which shows the shape of what is coming instead of the fact that something is.

Assistively, a spinning glyph is nothing at all. It is a decorative shape unless something says what it means, so mark the region it belongs to as a busy state while it is filling, and give the wait an accessible name through a role="status" line beside the glyph (“Loading your report”) rather than leaving a bare SVG to speak for itself. Clear both when the work lands, including on the error path, because a region left busy is a region assistive technology has stopped reading. And the animation is motion like any other: prefers-reduced-motion should slow or still it, since an endlessly rotating element is exactly the kind of thing that setting exists for.

The folk name is worth knowing. Early browsers put an animated logo in the corner that turned while the page was loading, and it was called the throbber, a word that survives in Mozilla’s own source and in enough old bug reports that you will meet it. Spinner, loading indicator, and Apple’s activity indicator are the current names for the same thing, all of them naming the absence of a number.

One collision to keep straight, because the words are a letter apart and mean nothing alike. A spinner reports a wait; a spinbutton is a number field with up and down arrows that the reader operates, and that control is filed here as the stepper, which owns the spinbutton name. If someone hands you a ticket for a spinner and the mock has arrows on it, they meant a stepper.