tertiary adj.

"1 more than two; the number which is one more than two; a symbol representing this number;" Old English þreo, fem. and neuter (masc. þri, þrie), from Proto-Germanic *thrijiz (source also of Old Saxon thria, Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse þrir, Danish tre), from nominative plural of PIE root *trei- "three" (source also of Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irish and Welsh tri "three").

3-D first attested 1952, abbreviation of three-dimensional (1878). Three-piece suit is recorded from 1909. Three cheers for ______ is recorded from 1751. Three-martini lunch is attested from 1972. Three-ring circus is recorded by 1898. Three-sixty "complete turnaround" is from 1927, originally among aviators, in reference to the number of degrees in a full circle. Three musketeers translates French les trois mousquetaires, title of the 1844 novel by Alexandre Dumas père.

"the third great geological period," 1841, Cainozoic, from Latinized form of Greek kainos "new, fresh, recent, novel" (see recent) + zōon "animal," but here with a sense of "life" (from PIE root *gwei- "to live"). The era that began with the demise of the dinosaurs and the rise of "recent" species and continues to the present; it also is known as the Tertiary. Compare Paleozoic, Mesozoic.

We observe that Lyell, in his geological works, even the most recent, uses the word Cainozoic instead of Coenozoic or Cenozoic. Why the propounder of the terms Eocene, Miocene, etc., should thus spell the word is incomprehensible. If he is right in it, then to be consistent he ought to say Eocain, Miocain, Pliocain, Post-pliocain; for all have the same root καινός. [American Journal of Sciences and Arts, 1873]

early 15c., "consisting of four parts," from Latin quaternarius "of four each, containing four," from quaterni "four each, by fours," from quater "four times," related to quattuor "four" (from PIE root *kwetwer- "four"). Also as a noun, "the number four" (mid-15c.), from Latin quaternarius.

In geological sense, with capital Q-, attested from 1843 in English, proposed 1829 by French geologist Jules Pierre François Stanislas Desnoyers (1800-1887) as name for "the fourth great epoch of geological time," but because it comprises only the age of man (now reckoned as the last 2.6 million years), and the other epochs are reckoned in the tens of millions of years, not all accepted it. Compare Tertiary.