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

I don't have the vocabulary to articulate the concept fully, but I will make a suggestion that may be helpful. When we attend a live performance our visual cues often overcome our sonic attentiveness. Try closing your eyes as you lisen to live music and allow yourself to get a "sonic image" of the instruments or voices. Also note the quality of the sound, timbre and pitch, as the instruments (voices) interact with the environment. I have found that using this kind of reference allows me to judge the veracity of an audio system. I agree with the above posts that in my experience tube amplifiers and pre-amps are better able to achieve this sense of dimensionality and space.

Air is the space around individual instruments. When you hear halos I would guess that you are exciting the angels :)

Detlof your explanation is interesting however I always referred to this effect as the natural decay. Sometimes when I listen to this effect and I certainly hear it in spades on some recordings, it appears the decay may be a bit longer than I note with live music as though there is some type of reverberant effect added to the recording. Even a reverberant hall doesn't seem to hang on to notes as long as certain recordings do. At least I can't recall hearing it to the same degree. Live seems a bit faster.

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.

Last Saturday I went to the symphony. I heard Beethoven.
I did not hear any air around any of the instruments in the orchestra, nor around the piano playing the piano concerto.
And 3 weeks ago I went to a 3 day folk music festival with all music played through a concert sound system, even the classical guitarists. I never heard any air around instruments there either.
What I did hear at both the Beethoven concert and the folk music festival was music that had a wholeness to it. The symphony was as one instrument with many many individual colors. And music through a sound system is completely as one, no matter where on the stage the person is playing.
I have often read the phrase "air around instruments" in reviews. But is this not just one of those "audiophile sound characteristics" rather than what music really sounds like?!

As with Sharri, I do not hear air around the instruments. After reading numerous posts on this subject I was ready to do a system change for this very reason. I have a soundstage and the speakers disappear, but would have a difficult time telling where a musician is on stage unless playing a solo, especially with a full orchestra piece. I can generally sense where the musicians are with small groups as in rock or jazz. I have recently been attending many concerts which featured jazz as well as classical music and noted exaclty as stated above, that in general the music comes to me as a cohesive whole.