qua·ter·nar·y 
(kwŏtər-nĕr′ē, kwə-tûrnə-rē)
(kwŏtər-nĕr′ē, kwə-tûrnə-rē)adj.
1. Consisting of four; in fours.
2. Quaternary Of, relating to, or being the period of geologic time from about 2.6 million years ago to the present, the more recent of the two periods of the Cenozoic Era. It is characterized by the appearance and development of humans and includes the Pleistocene and Holocene Epochs. See Table at geologic time.
3. Chemistry Relating to a nonhydrogen atom bonded to four other nonhydrogen atoms, especially to four carbon atoms: a quaternary nitrogen atom.
n. pl. qua·ter·nar·ies
1. The number four.
2. The member of a group that is fourth in order.
3. Quaternary The Quaternary Period.
[Latin quaternārius, from quaternī, by fours, from quater, four times; see kwetwer- in the Appendix of Indo-European roots.]