What is the name of the metal ring around the wheel hub of a bike?

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http://www.mountain-bike-buzz.com/images/mountain-bike-wheel-parts.jpg

I was wondering what is it called and if it serves any purpose besides being decorative.
I mean the concentric metal ring that surrounds the hub that is much smaller than the rim.

It isn't labeled.

Is it part of the hub?
 
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