quaternary

(noun) last 2 million years. Synonyms: Quaternary, Quaternary period, Age of Man.

(noun) the cardinal number that is the sum of three and one. Synonyms: four, 4, IV, tetrad, quatern, quaternion, quaternity, quartet, quadruplet, foursome, Little Joe.

(adj) consisting of or especially arranged in sets of four. Synonyms: quaternate. Examples: "Quaternate leaves." "A quaternary compound."

(adj) coming next after the third and just before the fifth in position or time or degree or magnitude. Synonyms: fourth, 4th. Example: "The quaternary period of geologic time extends from the end of the tertiary period to the present."