What is the meaning of Quaternary?

noun

• Last 2 million years.
  • synonyms: Quaternary period, Age of Man
• The cardinal number that is the sum of three and one.
  • synonyms: four, 4, IV, tetrad, quatern, quaternion, quaternity, quartet, quadruplet, foursome, Little Joe

adjective

• Consisting of or especially arranged in sets of four.
  • usage: "quaternate leaves"; "a quaternary compound"
  • synonyms: quaternate
• Coming next after the third and just before the fifth in position or time or degree or magnitude.
  • usage: "the quaternary period of geologic time extends from the end of the tertiary period to the present"
  • synonyms: fourth, 4th