| otosclerosis | (noun) hereditary disorder in which ossification of the labyrinth of the inner ear causes tinnitus and eventual deafness | - |
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| otoscope | (noun) medical instrument consisting of a magnifying lens and light; used for examining the external ear (the auditory meatus and especially the tympanic membrane) | Synonyms: auriscope, auroscope |
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| photosensitivity | (noun) sensitivity to the action of radiant energy | Synonyms: radiosensitivity |
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| photosphere | (noun) the intensely luminous surface of a star (especially the sun) | - |
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| photostat | (noun) a photocopy made on a Photostat machine | - |
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| photosynthesis | (noun) synthesis of compounds with the aid of radiant energy (especially in plants) | - |
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