'protozoa' originates from New Latin (Neo-Latin), specifically from the compound formed from Greek elements 'proto-' and 'zoon', where 'proto-' meant 'first' and 'zoon' meant 'animal'.

'protozoon' (singular) was formed from Greek roots and used in scientific Latin; the plural 'protozoa' entered English scientific usage in the 19th century as the name for a group of 'first' or 'primitive' animals.

Initially, it meant 'first animals' (i.e., primitive microscopic animals); over time, as biological understanding improved, the term came to denote a range of single-celled eukaryotes and the original taxonomic group was largely revised or abandoned.