With all due respect to John, who has done more than anyone else around to spread the good word about the Prius, "stealth" is a misnomer. At normal road speeds, tire noise is louder than engine noise on any modern car in good working order. In a parking lot, the Prius in electric mode may not make enough noise for pedestrians to notice, and makes no noise at all when you press the Power button to put it in ready mode.
The Prius, when in electric mode and moving above a few mph, is, at the very best, very slightly less noisy than a normal, modern, well-tuned car with a good muffler. This slightly-less-noisy state is termed "stealth mode" by some, and "electric mode" by others.
Prius sold in Europe and Japan comes with an EV button to induce this condition. The U.S. Prius does it when it feels like it, but can be coaxed into it, under the right conditions, by easily-learned techniques.
It isn't really, if you take all definitions into account.
The more common "silent" definition applies to the slower speeds.
The faster a vehicle travels, the less of a chance you get to hear it anyway, due to time & distance. That gives ambient noise an opportunity to "conceal", the other definition. So whether the tires make any sound or not doesn't make much of a difference.