what does "Air or a Halo around instruments" mean?

Well, I suppose it is a function of hearing acuity, perhaps also a question of having absolute pitch or not. I don't know. I listened to a lonely flute player in a Zurich park yesterday and the way his tones formed and carried had plenty of air. Afterwards I went home and listened to solo flute music, similar to what was played outside on this beautiful spring day and at least a third of the air was gone. "Air" is what lets the music spread in space and our stereos certainly have (indeed sometimes shockingly!) less of it. Especially as far as orchestral music on CDs, without upsampling are concerned.
I agree partly with Jetter, that you can listen to a musical interpretation as a cohesive whole. But then, I suppose its training, you can easily focus on a single, or a group of instruments, even in big orchestral music. What do you suppose conductors do, when in rehersals they form the sound of a given orchestra according to their ideas of interpretation? Even during a tutti, they have to differentiate their hearing to grasp what the horns, the celli, the woodwinds etc are doing and this goes down even to single players of whatever section. Cheers,

Moving air is what music is!
Most music reproduction systems tend to lose much of the "moving air" that is the music. The dynamics and clarity of live music just vanish. Ever listen to your system after coming home from the symphony??!! Does it sound like you are sitting in the foyer, or on the main concert floor?
A system that has "air around instruments" may have a bit of the dynamics and details of the live experience. And perhaps that's what this refers to.
But at a concert, I never hear "air around instruments" but I do hear a lot of moving air. To me, it seems like that is a difference. But maybe not.
BTW, I do not have perfect pitch or hearing.

Eantala, I interpret air to mean hearing the ambiance or environment into which the instrument is playing. For instance, you hear the oboe soloist not just playing notes to the left of center orchestra (localizing within the soundstage) but also projecting sound into the acoustic of the concert hall. You hear the sound of the instrument and the dispersal of the sound at the same time. I used to think this was just exageration or poetic nonsense. With a tube amp I finally heard and understood what they were talking about. It's one of those things you have to experience yourself to "get it". I never really heard it with solid state so never knew what I was missing.

I think "air around instruments" is sort of a mixed metaphor, if you will, so we end up talking about two different things here. Having a system reproduce voices clearly separated in space is a soundstaging feature, and one, IMO, that is not terribly realistic, as others have noted. Still, I'd rather have it than not. Soundstaging "tricks" are often very pleasurable surrogates for the natural spaciousness and grandeur (not to mention the visual aspect) of live music that most of us cannot hope to reproduce in our homes. I'm willing to be duped by good soundstaging.
Air, it seems to me, is not a soundstaging feature. It has to do with fine detail that captures the interior texture of sounds. I think I'm with Sharri here. A system that does "air" well conveys the sense that music is aural painting of air, that the sounds are illumined and alive on the inside. It's easier to think of what lack of "air" is: a lot of reproduced music gives dynamics, shape and surface texture to the sound, but ultimately the sounds are rather dense and inert on the inside. My four-bit words on the subject.