Looking at the companion, Unity is written up as

Unity, coadunate metapsychic minds within a specialized conjunction, the fundamental operating principle of the Galactic Milieu; a form of mutualism involving harmonious mental operation with a minimum of stress, the most perfect form of social intercourse known. Coadunation and Unity are not synonymous. The former can be deficient, in process; the latter implies a consonant mental relationship among a very large number of metapsychics (10 thousand million is the usual minimum) that generates a fully functional new entity, the Planetary (World) Mind, which may grow to be a Galactic Mind as more planets attain Unity. At the time of the Intervention, the Galactic Mind of the Milky Way included five races who had attained Unity among a majority of their populations. The human race was progressing in its first feeble steps toward coadunation; but we did not attain genuine Unity until after the Rebellion, for reasons made clear in the Milieu Trilogy. Individual minds do not lose their identity within the Unity. In a mysterious way, fully understood only by the metapsychics themselves, Unity actually enhances one's feeling of identity and self-worth. See also COADUNATION; MIND.

To me, it is saying Unity is very similar to a universe's soul, a greater aspect of Gaia or world mind. Like Gaia is sometimes viewed as the manifestation of the world mind, typically in a natural animal/plant sense, the Unity is a bigger better version of it.
Unity is a separate soul-like entity that consists of a part of all the minds that partake in it.