Ok, that's what they call it. The dictionary says the same, basically at least. I just still have the feeling, as I'm not an English native speaker perhaps, that a washer is something you put under a nut or a bolt's head.
This part on my bike at least looks polished and hardened, so I guess it's rather what leo calls a "thrust ring" or "sealing washer". One could call down the whole thing as anal-talk, I nevertheless find two things (at least) worth to be mentioned: it's usually not a cheap 20 cents washer and, if you compare it with the pictures, the inner diameter has to be bigger than 25mm. This ring, the one shown as #33, is part of the bearing, sitting ON the inner race, not in front of it. It's not part of the swingarm or the axle. That's why I doubt that these rings are missing on certain bikes, unless the AO or often berated PO has lost it.
Hubert