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