If something is quaternary, it's either the fourth in position or exists in a group of four. If you're the fourth child in your family, you can describe yourself as the quaternary sibling.

In geology, the Quaternary Period (also known as the Age of Humans) got its name from being the fourth named period of geologic time. Although scientists now classify these periods in a new way, and it's no longer the fourth, the Quaternary kept its name. In math, a base four numerical system is also called a quaternary. You'll almost always find this adjective in a technical or scientific context. It derives from the Latin quater, "four."

Definitions of quaternary
  1. adjective
    consisting of or especially arranged in sets of four
    “a quaternary compound”
    synonyms: quaternate
    having or involving or consisting of more than one part or entity or individual
  2. adjective
    coming next after the third and just before the fifth in position or time or degree or magnitude
    “the quaternary period of geologic time extends from the end of the tertiary period to the present”
    synonyms: 4th, fourth
    being or denoting a numerical order in a series
  3. noun
    the cardinal number that is the sum of three and one
    synonyms: 4, IV, Little Joe, four, foursome, quadruplet, quartet, quatern, quaternion, quaternity, tetrad
    see moresee less
    type of:
    digit, figure
    one of the elements that collectively form a system of numeration
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