What is a Servo Tonearm?

When I hear "servo tonearm" I think about this model:
https://www.vinylengine.com/library/denon/dp-51f.shtml
Never heard it but it looks cool...

I think the term applies to linear trackers that are motorized. The pivot is pushed across the LP surface by a motor that receives feedback based on the movement at the stylus tip. Thus such tonearms describe a succession of tiny arcs as they move. They’re actually not truly linear. The prototypical example was the Rabco.

A servo is any self-adjusting circuit. Notice the Denon table dweller links to is described as both servo drive and servo arm. Servo drive detects and corrects differences in speed. Servo arm, angles. 

The pros with the arm are they think a cheap circuit can do at least as well as more expensive to manufacture bearings. Also good for marketing to guys who love unnecessary gidgety gadgety gear. 

The cons are they were wrong, except about the gadget thing. We know they were wrong about performance, as otherwise they'd still be around and dominating. And we know they were right about the latter, as otherwise you wouldn't even be asking.

Servo tonearms are not new, they have been around for years. The most familiar are the vintage Rabco, Souther, and the T-3 and T-5 Goldmund arms. They look like fun but are notoriously difficult to set up and maintain. Notice that only mega-buck turntable manufacturers offer linear tracking servo arms these days. IMO, you are better served with a pivoting tonearm.