one-celled alga: a microscopic one-celled alga that has silica-filled cell walls or shells divided into two halves. Diatoms are responsible for the formation of diatomite in water. Class Bacillariophyceae.
[Mid-19th century. < modern Latin Diatoma< Greek diatomos "cut in two" < diatemnein "to cut through" < temnein "to cut"]
