| infrared | (adjective) having or employing wavelengths longer than light but shorter than radio waves; lying outside the visible spectrum at its red end | - |
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| infrasonic | (adjective) having frequencies below those of audible sound | - |
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| infrequent | (adjective) not frequent; not occurring regularly or at short intervals | - |
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| prefrontal | (adjective) anterior to a frontal structure | - |
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| refractile | (adjective) of or relating to or capable of refraction | Synonyms: refractive |
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| refractive | (adjective) of or relating to or capable of refraction | Synonyms: refractile |
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| (adjective) capable of changing the direction (of a light or sound wave) | Synonyms: deflective |
| refractory | (adjective) stubbornly resistant to authority or control | Synonyms: fractious, recalcitrant |
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| (adjective) temporarily unresponsive or not fully responsive to a process or stimulus | - |
| (adjective) not responding to treatment | Synonyms: stubborn |
| refreshed | (adjective) with restored energy | Synonyms: fresh, invigorated, reinvigorated |
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| refreshful | (adjective) imparting vitality and energy | Synonyms: bracing, brisk, fresh, refreshing, tonic |
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| refreshing | (adjective) imparting vitality and energy | Synonyms: bracing, brisk, fresh, refreshful, tonic |
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| (adjective) pleasantly new or different | Synonyms: novel |
| refrigerant | (adjective) causing cooling or freezing | Synonyms: refrigerating |
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| refrigerated | (adjective) made or kept cold by refrigeration | - |
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| refrigerating | (adjective) causing cooling or freezing | Synonyms: refrigerant |
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| suffrutescent | (adjective) of a plant; having a woody base that does not die down each year | - |
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| unafraid | (adjective) oblivious of dangers or perils or calmly resolute in facing them | Synonyms: fearless |
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| (adjective) free from fear or doubt; easy in mind | Synonyms: secure, untroubled |
| unframed | (adjective) not provided with a frame | - |
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| unfree | (adjective) hampered and not free; not able to act at will | - |
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| (adjective) held in servitude | - |
| unfrequented | (adjective) devoid of creatures | Synonyms: lonely, solitary |
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| unfretted | (adjective) without frets | - |
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| unfriendly | (adjective) not disposed to friendship or friendliness | - |
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| (adjective) not easy to understand or use | - |
| (adjective) very unfavorable to life or growth | Synonyms: hostile, uncongenial |
| (adjective) not friendly | Synonyms: inimical |
| unfrightened | (adjective) not affected by fright | - |
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| unfrosted | (adjective) (of glass) lacking a frosted coating | - |
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| unfrozen | (adjective) not frozen | - |
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| unfruitful | (adjective) not fruitful; not conducive to abundant production | - |
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| unrefreshed | (adjective) not rested or refreshed | Synonyms: unrested |
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| upfront | (adjective) frank and honest | - |
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| usufructuary | (adjective) of or relating to the nature of a usufruct | - |
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