adjective

1. consisting of four; in sets of four
2. designating or of the second and current geologic period of the Cenozoic Era, usually subdivided into the Pleistocene and Holocene epochs and characterized by a series of ice ages, the first toolmaking cultures, and the rise of modern human culture
3. Chem.
   1. designating a compound or alloy containing four different components or elements quaternary silver
   2. being, of, or containing an atom linked to four carbon atoms a quaternary nitrogen atom

Etymology: L quaternarius < quaterni, four together, four each < quater, four times < base of quattuor, four

noun pl. quaternaries -·nar′·ies (-nar′ies)

1. the number four
2. a set of four