| orthophosphate | (noun) a salt of phosphoric acid | Synonyms: inorganic phosphate, phosphate |
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| orthopnea | (noun) form of dyspnea in which the person can breathe comfortably only when standing or sitting erect; associated with asthma and emphysema and angina pectoris | - |
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| orthopter | (noun) heavier-than-air craft that is propelled by the flapping of wings | Synonyms: ornithopter |
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| orthopteran | (noun) any of various insects having leathery forewings and membranous hind wings and chewing mouthparts | Synonyms: orthopteron, orthopterous insect |
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| orthopteron | (noun) any of various insects having leathery forewings and membranous hind wings and chewing mouthparts | Synonyms: orthopteran, orthopterous insect |
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| orthoptic | (adjective) of or relating to normal binocular vision | - |
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| orthoptics | (noun) treatment of defects of binocular vision (such as strabismus and amblyopia) by nonsurgical measures (especially by exercises to strengthen the eye muscles) | - |
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| orthoptist | (noun) a specialist in orthoptics | - |
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| orthoscope | (noun) an ophthalmoscope with a layer of water to neutralize the refraction of the cornea | - |
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| orthostatic | (adjective) pertaining to an upright standing posture | - |
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| orthotropous | (adjective) (of a plant ovule) completely straight with the micropyle at the apex | - |
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| ortolan | (noun) brownish Old World bunting often eaten as a delicacy | Synonyms: Emberiza hortulana, ortolan bunting |
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| ortygan | (noun) any of several East Indian birds | - |
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| oryx | (noun) large African antelope with long straight nearly upright horns | Synonyms: pasang |
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| orzo | (noun) pasta shaped like pearls of barley; frequently prepared with lamb in Greek cuisine | - |
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