Diatom

n.	1.	(Bot.) One of the Diatomaceæ, a family of minute unicellular Algæ having a siliceous covering of great delicacy, each individual multiplying by spontaneous division. By some authors diatoms are called Bacillariæ, but this word is not in general use.
	2.	A particle or atom endowed with the vital principle. The individual is nothing. He is no more than the diatom, the bit of protoplasm. - Mrs. E. Lynn Linton.

Noun

1.

diatom - microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica

alga, algae, Bacillariophyceae, class Bacillariophyceae, class Diatomophyceae, Diatomophyceae, phytoplankton