| enigma | (noun) something that baffles understanding and cannot be explained | Synonyms: closed book, mystery, secret |
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| (noun) a difficult problem | Synonyms: brain-teaser, conundrum, riddle |
| sigma | (noun) the 18th letter of the Greek alphabet | - |
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| stigma | (noun) a skin lesion that is a diagnostic sign of some disease | - |
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| (noun) an external tracheal aperture in a terrestrial arthropod | - |
| (noun) a symbol of disgrace or infamy | Synonyms: brand, mark, stain |
| (noun) the apical end of the style where deposited pollen enters the pistil | - |