What is the correct name for this tightening nut on my tablet holder?

[fr3d]

I lost it and need to get another one. Maybe there's a 3d print model I can use, but I don't know what the proper name of this is.

 

[4wd]

I think it's a knurled tightening nut ...
You could ask where you bought it for a spare.
I have noticed these ram mount style fittings are all very similar so if you have another you can borrow from elsewhere it *might* fit.
They are widely used in all kinds of cheap phone and tablet holders - have a look on ebay for a cheap one to cannibalise.

 

[TheLightSpeed!]

Spherical Socket Friction nut; otherwise known in the community as a "SSFN".