Webster's Revised Unabridged Dictionary · 1913

Unicity

Unicity , noun

[Latin unicus single. See Unique.]

The condition of being united; quality of the unique; unification.
Not unity, but what the schoolmen call unicity. — De Quincey
The unicity we strive not to express, for that is impossible, but to designate by the nearest analogy. — Coleridge