<p>How should I describe this part which looks like a small gear so that I can research replacements?</p>

<p><a href="https://i.stack.imgur.com/51BdD.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/51BdD.jpg" alt="image of a pulley"></a></p>

<p>This came with my FLSUN 3D printer, which may be based on a Prusa design.</p>

It is an "aluminum timing  pulley"
https://www.google.com/search?psb=1&tbm=shop&q=aluminum%20timing%20pulley&ved=0CAMQr4sDKAFqFwoTCMis1KHmiuMCFRoMswAdMqUElxAB
https://www.ebay.com/i/152446519860?chn=ps&var=453435947176