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

Abstract7 is right to my mind. If you listen for example to a solo violin or a solo piano very carefully, you will notice, how the tone appears and spreads in the soundspace with its overtones forming a cluster around its tonal core, if you like. If you were to translate this into colour, it would, for example be like a strong red at its central core, which then would spread out, like ripples from a stone thrown into a pond, into different hues of red, getting weaker and weaker in intensity. It is how the tone, or a succession of tones, spead and decay after their first appearance, which is called the halo. If a system reproduces this well, it has , as Abstract has suggested, air. Electrostatics do this quite nicely, though still far from the real thing.

Tubegroover, I quite agree with your point, but found it difficult to differentiate the halo-effect from what is described as natural decay, since the halo will decay together with the tone per se. In a live offering, easily to hear with chamber music in a smallish hall, or with a solo instrument played in an ordinary room, you can easily discern the halo forming around a note played, which can be quite startling, if you've listened to canned music for a long time, because I know of no system, which is able to reproduce this effect properly. It is either missing entirely, or truncated, or as you so rightly suggest, simply too slow. Live certainly is faster, I agree, also more compact as a cluster around the central tone.

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,