| Term | Definition |
| ultrasound | using the reflections of high-frequency sound waves to construct an image of a body organ |
| ultraviolet | relating to electromagnetic radiation with frequencies higher than those of visible light |
| ultraist | a person who pushes a principle to extremes |
| ultrasonic | having frequencies above audible sound |
| ultramarine | of a brilliant pure blue to purplish blue color |
| ultramicroscopic | too tiny to be seen with an ordinary microscope |
| ultrastructure | the detailed structure of a cell, tissue, or organ, that can be observed only by electron microscopy |
| ultramicrobalance | a balance for weighing precisely, to a hundredth of a microgram or less, minute qualities of material |
| ultralight | extremely lightweight in comparison of others of its kind |
| ultrametamorphic | a time during which the temperature of a rock exceeds its melting point |