| rho | (noun) the 17th letter of the Greek alphabet | - |
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| rhodanthe | (noun) Australian annual everlasting having light pink nodding flower heads; sometimes placed in genus Helipterum | Synonyms: Helipterum manglesii, Rhodanthe manglesii, Swan River everlasting |
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| rhodium | (noun) a white hard metallic element that is one of the platinum group and is found in platinum ores; used in alloys with platinum | Synonyms: atomic number 45, Rh |
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| rhodochrosite | (noun) a mineral consisting of manganese carbonate; a source of manganese | - |
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| rhododendron | (noun) any shrub of the genus Rhododendron: evergreen shrubs or small shrubby trees having leathery leaves and showy clusters of campanulate (bell-shaped) flowers | - |
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| rhodolite | (noun) a red or pink variety of garnet used as a gemstone | - |
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| rhodomontade | (noun) vain and empty boasting | Synonyms: bluster, braggadocio, rodomontade |
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| rhodonite | (noun) a pink or red mineral consisting of crystalline manganese silicate; used as an ornamental stone | - |
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| rhodopsin | (noun) a red photopigment in the retinal rods of vertebrates; dissociates into retinene by light | Synonyms: retinal purple, visual purple |
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| rhomb | (noun) a parallelogram with four equal sides; an oblique-angled equilateral parallelogram | Synonyms: diamond, rhombus |
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| rhombencephalon | (noun) the posterior portion of the brain including cerebellum and brainstem | Synonyms: hindbrain |
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| rhombohedron | (noun) a parallelepiped bounded by six similar faces (either rhombuses or parallelograms) | - |
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| rhomboid | (noun) any of several muscles of the upper back that help move the shoulder blade | Synonyms: rhomboid muscle |
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| (noun) a parallelogram with adjacent sides of unequal lengths; an oblique-angled parallelogram with only the opposite sides equal | - |
| rhombus | (noun) a parallelogram with four equal sides; an oblique-angled equilateral parallelogram | Synonyms: diamond, rhomb |
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| rhonchus | (noun) a sound like whistling or snoring that is heard with a stethoscope during expiration as air passes through obstructed channels | - |
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