Origins
The word 'centennial' entered English in the eighteenth century as a compound of Latin elements: 'centum' (hundred) and 'annus' (year), modeled on existing English words like 'biennial' and 'perennial.' It means 'of or relating to a hundredth anniversary' and, as a noun, 'a hundredth anniversary or its celebration.' The variant 'centenary' (from Latin 'centenārius') carries the same meaning and is more common in British English.
The Latin 'centum' (hundred) descends from PIE *ḱm̥tóm, which is itself a derivative of *deḱm̥ (ten). The hundred was conceived as 'a great ten' — ten multiplied by ten, the next order of magnitude. This derivation is preserved across the Indo-European family: Latin 'centum,' Greek 'hekatón,' Sanskrit 'śatám,' Lithuanian 'šim̃tas,' Old Irish 'cét,' and, through Germanic, English 'hundred' (from Proto-Germanic *hundą, from PIE *ḱm̥tóm, with a suffix meaning 'counted number').
The word 'centum' is also famous in historical linguistics for giving its name to the 'centum-satem division' — the classification of Indo-European languages into two groups based on how they treated PIE palatal stops. In 'centum' languages (Latin, Greek, Celtic, Germanic), the PIE palatal *ḱ became a plain velar /k/. In 'satem' languages (Sanskrit, Avestan, Slavic, Baltic), it became a sibilant /s/ or /ʃ/. The word for 'hundred' — Latin 'centum' versus Avestan 'satəm' — provided the labels for this classification.
Latin Roots
The word family from 'centum' is extensive. 'Century' (a hundred years, or a Roman military unit of roughly a hundred men), 'centurion' (commander of a century), 'cent' (one hundredth of a dollar), 'percent' (per hundred), 'centigrade' (divided into a hundred degrees), 'centimeter' (one hundredth of a meter), 'centipede' (hundred-footed, though centipedes never have exactly a hundred legs), and 'bicentennial' (two hundredth anniversary).
The celebration of centennials became culturally significant in the modern era. The United States Centennial Exhibition of 1876, held in Philadelphia, celebrated the hundredth anniversary of the Declaration of Independence and showcased American industrial achievement. It was the first official World's Fair held in the United States and attracted nearly ten million visitors. The American Bicentennial of 1976 was an even larger national celebration.
The concept of the centennial reflects a particular relationship with historical time. To celebrate a centennial is to assert continuity — to claim that the entity being celebrated (a nation, an institution, a city) has persisted for a hundred years and that this persistence matters. It is both a backward look (what happened a century ago) and a forward claim (what will endure for the next century). The word packages historical consciousness into a single compound.
Modern Legacy
The sequence of anniversary terms — bicentennial (200), sesquicentennial (150), centennial (100), semicentennial (50), quadranscentennial (25) — demonstrates the precision of Latinate compound vocabulary. Each term is built from transparent Latin elements that specify the exact number of years. This system can extend indefinitely: a tercentennial (300 years), a quadricentennial (400 years), a quincentennial (500 years), and so on. The vocabulary is modular, each prefix snapping onto the '-ennial' base like a numerical dial.