Quaternary

1. fourth in order or rank; belonging to the fourth order.
2. relating to or denoting the most recent period in the Cenozoic era, following the Tertiary period and comprising the Pleistocene and Holocene epochs.
3. denoting an ammonium compound containing a cation of the form NR₄⁺, where R represents organic groups or atoms other than hydrogen.

noun, singular proper noun

the Quaternary period or the system of deposits laid down during it.

late Middle English (as a noun denoting a set of four): from Latin quaternarius, from quaterni ‘four at once’, from quater ‘four times’, from quattuor ‘four’.