un

The root un-/uni- comes from Latin ūnus, meaning simply "one." It is the same "one" you can still hear inside English unit, French un, Spanish uno, and Italian uno. Because counting starts at one, this root sits at the foundation of an entire family about singleness, oneness, and being joined into a single whole.

Start with the plainest member: a unit is literally "one thing" — one item, one measurement, one apartment in a building. From there the family branches in two directions.

The first direction is making many into one. Add a verb ending and ūnus becomes unite — to bring separate things together so they act as one. The noun is union (a joining into one) and the state is unity (the condition of being one). Latin even built a verb unificare, "to make one," which gives English unify and its noun unification — the hidden -fic- there is the root fac "to make," so unify is literally "to make one." Add re- (again) and you reunite people after a separation, holding a reunion.

The second direction is one as a quality of a single thing. Latin unicus meant "the only one of its kind," and that became unique. Combine uni- with other roots and you get a whole row of "one-" compounds, each transparent once you spot the second root:
- uni- + form (shape) → uniform: one form, applied to everyone
- uni- + lateral (side) → unilateral: one-sided, done by one party alone
- uni- + son (sound) → unison: one sound, many voices on one note
- uni- + corn (horn) → unicorn: a one-horned creature
- uni- + cycle (wheel) → unicycle: a one-wheeled vehicle
- uni- + sex → unisex: one style for both sexes

Two members hide ūnus inside a longer word. Unanimous is unus + anim (mind) — "of one mind," complete agreement; the un- here is "one," not the negative "not," which is the single most useful fact about the word. And universe is the most surprising of all: uni- + vers (from vertere, "to turn") = universum, "everything turned into one" — the Romans' name for the whole of existence rolled into a single thing. From universe came universal (applying to the one whole = to everything) and even university (originally a whole community of teachers and students taken as one body).

The rule across the whole family is steady: wherever you see uni- (or un- before a mind/spirit word), think "one," and the second root tells you one what — one form, one side, one sound, one mind.