Merriam-Webster Collegiate Dictionary - quaternary

 I. adjective  Etymology: Latin quaternarius, from quaterni four each  Date: 1605  1.  a. of, relating to, or consisting of four units or members  b. of, relating to, or being a number system with a base of four  2. capitalized of, relating to, or being the geological period from the end of the Tertiary to the present time or the corresponding system of rocks — see geologic time table  3. consisting of, containing, or being an atom bonded to four other atoms  II. noun  (plural -ries)  Date: 1880  1. capitalized the Quaternary period or system of rocks  2. a member of a group fourth in order or rank