Neck muscles, skull size, jaw shape and density, internal workings (e.g. Superior protective measures inside the head and body). A fighter's ability to endure punishment without being rocked or knocked out is sometimes reduced as a fighter ages, suffers concussions, or generally takes a lot of damage through their career. There's enough anecdotal and visual evidence to show this is most likely the case. That's the chin in a physiological sense. It definitely exists, but assessment of chin strength is fraught with difficulties because of the infinite number of variables as a witness of a bout in only a semi-controlled environment.
But other characteristics may contribute to the appearance of a physologically superior chin. Some fighters may be particularly good at rolling with punches and weakening the impact of their opponent's shots, while others might have the conditioning (physiological?) Or mental fortitude/fighting will to continue physically exerting whilst enduring punishment or 'spring back' after being rocked. Discering whether it is a fighter's ability to defend against being knocked out or to defend against being rocked that is considered to represent a fighter's 'chin' might help narrow down the criteria. I'm not sure on which side the MMA community leans.
Its by no means an exact science at the moment - and its easy to make misjudgements about a fighter's chin - but it is still possible to have reasonable discussion about the subject.
Last edited by JaPo; 10-31-2013 at 08:00 PM.