Definitions of Quaternary:

• noun:   last 2 million years
• noun:   the cardinal number that is the sum of three and one
• adjective:   consisting of or especially arranged in sets of four

  Example: "A quaternary compound"

• adjective:   coming next after the third and just before the fifth in position or time or degree or magnitude

  Example: "The quaternary period of geologic time extends from the end of the tertiary period to the present"