| acantha | (noun) any sharply pointed projection | Synonyms: spine, spur |
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| acanthion | (noun) the craniometric point at the anterior extremity of the intermaxillary suture | - |
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| acanthocephalan | (noun) any of various worms living parasitically in intestines of vertebrates having a retractile proboscis covered with many hooked spines | Synonyms: spiny-headed worm |
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| acanthocyte | (noun) an abnormal red blood cell that has thorny projections of protoplasm | - |
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| acanthocytosis | (noun) the presence of acanthocytes in the blood stream (as in abetalipoproteinemia) | - |
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| acantholysis | (noun) a breakdown of a cell layer in the epidermis (as in pemphigus) | - |
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| acanthoma | (noun) a neoplasm originating in the epidermis | Synonyms: skin tumor |
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| acanthopterygian | (noun) a teleost fish with fins that are supported by sharp inflexible rays | Synonyms: spiny-finned fish |
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| acanthosis | (noun) an abnormal but benign thickening of the prickle-cell layer of the skin (as in psoriasis) | - |
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| acanthus | (noun) any plant of the genus Acanthus having large spiny leaves and spikes or white or purplish flowers; native to Mediterranean region but widely cultivated | - |
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| barbacan | (noun) a tower that is part of a defensive structure (such as a castle) | Synonyms: barbican |
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| coelacanth | (noun) fish thought to have been extinct since the Cretaceous period but found in 1938 off the coast of Africa | Synonyms: Latimeria chalumnae |
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| coracan | (noun) East Indian cereal grass whose seed yield a somewhat bitter flour, a staple in the Orient | Synonyms: African millet, corakan, Eleusine coracana, finger millet, kurakkan, ragee, ragi |
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| heterostracan | (noun) extinct jawless fish with the anterior part of the body covered with bony plates; of the Silurian and Devonian | - |
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| keratoacanthoma | (noun) skin tumor that grows rapidly (especially in older people) and resembles a carcinoma but does not spread; it usually disappears spontaneously, often leaving a scar | - |
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| osteostracan | (noun) extinct jawless fish of the Devonian with armored head | Synonyms: cephalaspid |
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| pyracanth | (noun) any of various thorny shrubs of the genus Pyracantha bearing small white flowers followed by hard red or orange-red berries | Synonyms: fire thorn, firethorn, Pyracantha |
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| tragacanth | (noun) a gum used in pharmacy, adhesives, and textile printing | - |
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| vacancy | (noun) an empty area or space | Synonyms: emptiness, vacuum, void |
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| (noun) being unoccupied | - |