(Science: organism) A very diverse group comprising some 50,000 eukaryotic organisms that consist of one cell. Because most of them are motile and heterotrophic, the Protozoa were originally regarded as a phylum of the animal kingdom.
However it is now clear that they have only one common characteristic, they are not multi cellular and Protozoa are now usually classed as a Sub Kingdom of the Kingdom Protista. On this classification the Protozoa are grouped into several phyla, the main ones being the Sarcomastigophora (flagellates, heliozoans and amoeboid like protozoa), the Ciliophora (ciliates) and the Apicomplexa (sporozoan parasites such as Plasmodium).
That is from our site’s dictionary. When protozoans were included in the animal kingdom, the kingdom had 2 phylums: protozoans and metazoans. Metazoans is now the same thing as animals, back then it meant pluricelullar animals.