| undulous | Resembling waves. |
| unfavorable | Adverse. |
| ungainly | Clumsy. |
| unguent | Any ointment or lubricant for local application. |
| unicellular | Consisting of a single cell. |
| univalence | Monovalency. |
| unify | To cause to be one. |
| unique | Being the only one of its kind. |
| unison | A condition of perfect agreement and accord. |
| unisonant | Being in a condition of perfect agreement and accord. |
| Unitarian | Pertaining to a religious body that rejects the doctrine of the Trinity. |
| unlawful | Illegal. |
| unlimited | Unconstrained. |
| unnatural | Artificial. |
| unnecessary | Not essential under the circumstances. |
| unsettle | To put into confusion. |
| unsophisticated | Showing inexperience. |
| unspeakable | Abominable. |
| untimely | Unseasonable. |
| untoward | Causing annoyance or hindrance. |
| unutterable | Inexpressible. |
| unwieldy | Moved or managed with difficulty, as from great size or awkward shape. |
| unwise | Foolish. |
| unyoke | To separate. |
| up-keep | Maintenance. |
| upbraid | To reproach as deserving blame. |
| upcast | A throwing upward. |
| upheaval | Overthrow or violent disturbance of established order or condition. |
| upheave | To raise or lift with effort. |
| uppermost | First in order of precedence. |
| uproarious | Noisy. |
| uproot | To eradicate. |
| upturn | To throw into confusion. |
| urban | Of, or pertaining to, or like a city. |
| urbanity | Refined or elegant courtesy. |
| urchin | A roguish, mischievous boy. |
| urgency | The pressure of necessity. |
| usage | Treatment. |
| usurious | Taking unlawful or exorbitant interest on money loaned. |
| usurp | To take possession of by force. |
| usury | The demanding for the use of money as a loan, a rate of interest beyond what is allowed by law. |
| utilitarianism | The ethical doctrine that actions are right because they are useful or of beneficial tendency. |
| utility | Fitness for some desirable practical purpose. |
| utmost | The greatest possible extent. |
| vacate | To leave. |
| vaccinate | To inoculate with vaccine virus or virus of cowpox. |
| vacillate | To waver. |
| vacuous | Empty. |
| vacuum | A space entirely devoid of matter. |
| vagabond | A wanderer. |