What is meant by a Diatom?

Diatoms are microscopic, unicellular, marine or freshwater algae of the phylum Chrysophyta, having cell walls containing silica.

Diatoms are used for filtration. The fine structures of diatom shells trap foreign particles in fluids, such as dirt, lint, hair and some other microscopic organisms. Therefore, Diatoms are often used to filter water, particularly water in hot tubs and swimming pools.