«اتِّحَادٌ فِي التَّصَوُّرِ»
A kind of unity within conception.
When particular circumstances and external individuating features are abstracted away, two distinct objects can be conceived under a common universal. Zayd and ʿAmr remain different persons, but both fall under the conception of humanity.
The unity is not their numerical identity as external individuals. It is an intellectual unity of what is conceived. Thus rational linkage should not be reduced to repeating a word.