The word 'triennial' is a direct construction from Latin components, entering English in the mid-16th century. It combines 'tri-', a prefix denoting "three", with '-ennial', a derivative of the Latin 'annus' (year). The '-ennial' element itself is seen in other temporal words like 'biennial' and 'centennial'. The literal meaning, "pertaining to a three-year period", has remained stable, logically describing events, publications, or terms that recur or last for three years. This pattern of numeral + temporal root is a consistent and productive method in English for forming precise calendrical terms.

Structure: tri (three) + enn (year) + ial (adjective suffix) - tri: Latin origin, from "tres" meaning "three". Functional role: numeral prefix. - enn: Latin origin, from "annus" meaning "year". The spelling 'enn' is a combining form of 'annus', adapted for English spelling. - ial: Latin suffix "-ialis", forming adjectives meaning "pertaining to" or "characterized by".