Tertius

Tertius is a Latin adjective and masculine noun meaning “the third.” In classical Latin it appears in masculine form tertius, feminine tertia, and neuter tertium, and it declines like a standard second-declension adjective. As an ordinal, tertius denotes the third item in a sequence and agrees with the noun it modifies in gender, number, and case. Common examples include phrases such as tertius liber (the third book) and tertia hora (the third hour).

In Latin usage, tertius serves to distinguish items positioned after primus (first) and secundus (second) in lists, generations, or other ordered sets. The word appears in literary, legal, and religious texts where authors label successive elements, chapters, days, or other units of time and order.

The stem tert- has given rise to a wide range of modern terms in English and other languages. In science and everyday language, tertiary is widely used to indicate a third level or stage, such as in tertiary education, tertiary care, or tertiary structure in biochemistry (the three-dimensional folding of a protein). In geology, the historic term “Tertiary” was once used to describe a broad time interval, now subdivided into the Paleogene and Neogene periods. In taxonomy, tertius or tertia may appear as an epithet in binomial names to signal a third species described within a genus, though the practice is largely historical.

Today, tertius also appears as a personal name in some cultures, reflecting the Latin practice of using ordinals as given names. Overall, tertius remains a foundational Latin term that underpins numerous linguistic and scientific terms indicating the third position in a sequence.