What is this hose reel part called?
A swivel or &quot;rotary union&quot; (but there are many things under the same names that don't look like that, or in some cases work like that. It's imprecise, but does cover what you have, as well as some other things.)
Specifically one with one side &quot;garden hose thread&quot; and the other appears likely to be pipe thread in a smaller size.