We often say about transparency of sound.

What is in technical terms? Or it only subjective characteristic fo our ears?

Certainly we can to measure the transparency.

Transparency is complex of characteristics, which define the capability of non-distorted throughput for device.

If to say simply: Transparency = minimum harmonic distortions + flat frequency characteristic (less +/- 1 dB in declared band).

Usually we apply term "transparency" about hi-frequencies (5-10 kHz). We qualify it as "air" or "airy sound". What for low frequencies?

The same. But here speak not about "air", and about clarity of low frequencies. Bass notes don't merged. No bass hum.

What is harmonic distortion?

We send sine signal to input of device. At output we have this sine + other sines with other frequencies (named as harmonics). Other sines is harmoniс distortions.

About interaction of speakers and listening room in next post...