noun: the act of becoming formally connected or joined

Ramsay
Why do we compound? Because it produces variety, and variety is one of the aspects of the beautiful; Nature loves and abounds in variety, without violation of her unity. And further, all creation throbs with sympathy, one thing feeling and tending toward another, nothing content in isolation; and compound chords are chords reaching out after assimilation to an affiliation with other adjacent chords, that they may be able, through something in [Scientific Basis and Build of Music, page 70]

less variety of effect than we find in the diatonic chords; for although these chords may appear with their notes diversely named, there are still only the three. On account of their cosmopolitan character they need, and they have, no compounding with anything else. They are themselves at home everywhere; like a universal joint, they can turn any way, and affiliate in all directions. Being 4-note chords, and all of minor thirds, their effect is always minor, and they fall with loving softness to the diatonic chords to which they resolve. How this chord in its germ is found in the diatonic chord-scale; how it becomes a 4-note chord of minor thirds; how it variously resolves, each one of the three, in three manners with 24 tonic chords - all this is so fully set forth in the pre-note and treatise on the chromatic chord that it need not be more discussed in this place. See also Plates XVI., XVII., XVIII., XIX., and XX. [Scientific Basis and Build of Music, page 73]

See Also

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association
Sympathetic Association
sympathization
Sympathetic Sympathizer