A multi-celled animal plankton which has a food ingestion tube with an area of strong cilia whose motion gives the appearance of a rapidly revolving wheel. From the Latin, rotatus, "wheel".

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by John Harris in 1696 (Hudson and Gosse, 1886). Leeuwenhoek is mistakenly given credit for being the first to describe rotifers but Harris had produced sketches in 1703.