"eye doctor," 1610s, from French oculiste (16c.), from Latin oculus "an eye" (from PIE root *okw- "to see"). Middle English had oculister (early 15c.) "an authority on the eye and treatment of eye diseases."
offspring of a white and a mulatto, 1760, from Spanish *terceron, from tercero "a third (person)," from tercio "third," from Latin tertius "a third," from root of tres "three" (see three). So called from being third in descent from a Negro.
1650s, "of the third order, rank, degree, etc.," from Latin tertiarius "of or pertaining to a third," from tertius "third, a third," from root of tres "three" (see three). The geological sense (with capital T-) of "era after the Mesozoic" (which formerly was called the Secondary) is attested from 1794, after Italian terziari, used in this sense 1760 by Italian geologist Giovanni Arduino (1714-1795).
something indeterminate between two other things, 1724, Latin, literally "third something," from tertius "third, a third," from the root of tres "three" (see three). A loan-translation of Greek triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones." The Latin word also figures in phrases tertium non datur "no third possibility exists," and tertius gaudens "a third party that benefits from conflict between the other two."