1. (n.) The number four; a collection of four things; a quaternion.

2. (n.) A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad.

Euglena adenovirus aerobe aerobic bacteria amoeba anaerobe anaerobic bacteria animalcule bacillus bacteria bacterium bug coccus colon bacillus diatom disease-producing microorganism dyad echovirus enterovirus filterable virus flagellate fungus germ gram-negative bacteria gram-positive bacteria microbe microorganism microspore microzoa mold monad nematode nonfilterable virus paramecium pathogen picornavirus protozoa protozoon reovirus rhinovirus rickettsia salmonella saprophyte spirillum spirochete spore sporozoon staphylococcus streptococcus tetrad triad trypanosome vibrio virus volvox vorticellum zoospore