Webster's English Dictionary

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diatom

di.a.tom \'di--*-.ta:m\ n [deriv. of Gk diatomos cut in half, fr. 
   diatemnein to cut through,]fr. dia- + temnein to cut : any of a class 
   (Bacillariophyceae) of minute planktonic unicellular or c olonial algae 
   with silicified skeletons that form diatomite