Uni- means one

unicellular (adj) - having only one cell

unicycle (n) - a one-wheeled vehicle on which the rider sits and pedals

unidirectional (adj) - moving in only one direction

unify (v) - to join together into one group

unilateral (adj) - one sided

unique (adj) - one of a kind; unusual or rare

unison (n) - an instance of saying the same words or sounds at the same time; a group that sounds like one

unit (n) - one group

unitard (n) - a one-piece leotard and tights combination

unicorn (n) - a horse-like fabled animal that has one horn growing out of the middle of its forehead