| Term | Definition |
| unicellular | having only one cell. |
| unicorn | a horse-like fabled animal that has one horn growing out of the middle of its forehead. |
| unicycle | a one-wheeled vehicle on which the rider sits and pedals. |
| unidirectional | moving in only one direction. |
| unify | to join together into one group. |
| unilateral | one sided. |
| unique | one of a kind;unusual or rare. |
| unison | an instance of saying the same words or sounds at the same time;a group that sounds like one. |
| unit | one group. |
| unitard | a one-piece leotard and tights combination. |