What is an Action in modern React?

Direct answer

An Action is a function invoked within a Transition to perform work such as an asynchronous mutation while React coordinates pending state, optimistic UI, errors, and related rendering.

RenameButton.jsx

function RenameButton({ saveName }) {
  const [isPending, startTransition] = useTransition();

  function rename() {
    startTransition(async () => {
      await saveName("Ada");
    });
  }

  return (
    <button disabled={isPending} onClick={rename}>
      Rename
    </button>
  );
}

Modern React APIs such as useActionState() and useOptimistic() are designed to compose around this Action model.