Any of diverse minute acellular or unicellular organisms usually nonphotosynthetic WordNet 3.0 & Open English WordNet

Any of the diverse group of eukaryotes, of the phylum Protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia. Wiktionary

Any of a diverse group of eukaryotes. Wiktionary (translation)