any microscopic unicellular alga of the phylum Bacillariophyta, occurring in marine or fresh water singly or in colonies, each cell having a cell wall made of two halves and impregnated with silica
See also diatomite
Word origin
C19: from New Latin Diatoma (genus name), from Greek diatomos cut in two, from diatemnein to cut through, from dia- + temnein to cut
diatom in American English
(ˈdaɪɛˌtɑm; ˈdaɪətəm)
noun
any of a class (Bacillariophyceae) of microscopic algae (division Chromophycota), one-celled or in colonies, whose cell walls consist of interlocking parts and valves and contain silica: diatoms are a source of food for all kinds of marine life