n.
Function: adjective
Etymology: Latin tertiarius of or containing a third, from tertius third
Date: circa 1656
1 a : of third rank, importance, or value b chiefly British : of, relating to, or being higher education c : of, relating to, or constituting the third strongest of the three or four degrees of stress recognized by most linguists (as the stress of the third syllable of basketball team )
2 capitalized : of, relating to, or being the first period of the Cenozoic era or the corresponding system of rocks marked by the formation of high mountains (as the Alps, Caucasus, and Himalayas) and the dominance of mammals on land ― see GEOLOGIC TIME table
3 a : involving or resulting from the substitution of three atoms or groups <a tertiary salt> < tertiary amine> b : being or containing a carbon atom having bonds to three other carbon atoms <an acid containing a tertiary carbon> < tertiary alcohols> c : of, relating to, or being the normal folded structure of the coiled chain of a protein or of DNA or RNA
4 : occurring in or being a third stage: as a : being or relating to the recovery of oil and gas from old wells by means of the underground application of heat and chemicals b : being or relating to the purification of wastewater by removal of fine particles, nitrates, and phosphates