Octodecimus

Part of speech: adjective

Definitions

1. A term referring to the ordinal number representing the position of eighteenth in a sequence
2. Denoting the number that comes after seventeen and before nineteen, often used in mathematical contexts
3. Characterizing the position in a list or series that is occupied by the eighteenth item or element

Etymology: "Octodecimus" is the Latin ordinal for eighteenth, built from "octo" (eight) and "decimus" (tenth), a pattern that reads as "eight-and-ten" in the way "duodecimus" (twelfth) and "undecimus" (eleventh) do. Latin counted teens by adding the units to ten, so the sum-based construction is the same logic behind English "eighteen," which was formed in Old English as "eahtatiene," meaning "eight over ten." The word is a classical Latin form, so it appears in Roman-era texts and in the vocabulary of later Latin writers, but it never became a common English word. Its English descendants are mostly technical, such as "octodecimo," a book format where each sheet is folded into eighteen leaves, which is where the word survives in print-trade usage.

Synonyms: eighteenth, 18th

Antonyms: seventeenth, nineteenth, 17th, 19th