Aura, as is defined for this MUSH, is simply put, what makes a player character or any other special character different from non-player characters, cannon fodder, "redshirts", and the like. OOCly, the term is derived from the phrase "main character aura" - and unsurprisingly it fills a similar role here, except it's IC.
Just what IS Aura, though? In short, it SEEMS to be a slight preturbation in the field of reality linked to one's soul. However, beyond that, there seems to be absolutely no real concrete knowledge of its reality. However, it seems to be linked directly to someone's "importance" within their own world and the greater scheme of things, as well as one's power level, both in their world and outside. It is possible - though not entirely certain - that aura is merely a reflection of the power level the individual has. However, as Aura is so poorly understood, this too is impossible to verify.
Although everyone has at least SOME aura, chances are the more important or powerful you were, the more likely that you'll have more of it. It more or less correlates to one's power level, stat-wise. Aura is also what appears to allow people with it to survive otherwise impossible odds, withstand tremendous amounts of physical duress, battle otherwise insane odds and win, resist or recover otherwise crippling effects (such as mind control, mental reprogramming, etc.), and even when resistance fails being able to overcome what is supposedly impossible to cure.
There is VERY little known about Aura by the 'general public' of the MUSH. Founders can sense it in certain people, but that's only a vague notion. Only a select few Founders - those working directly for the Councils of the Union and Confed - know how to truly read someone's Aura in any sense other than the vaguest. The Councils know more, of course, but even their knowledge is limited, and closely guarded.