centum

Centum is indeclinable. One form for every case and gender, so centum viri, centum virorum and centum viris all look the same and the noun carries the grammar on its own. That makes it the last of the easy numerals: from two hundred upwards they decline as first and second declension plurals - ducenti, ducentae, ducenta, then trecenti, quadringenti and so on.

It sits under a good deal of vocabulary. Centurio is the officer over a hundred men, centuria a voting unit at Rome, and per centum is what English squashed into per cent. Comparative philology named half the Indo-European family after it: the centum languages, Latin among them, kept the hard c where the satem languages softened it. English hundred is the Germanic cousin of the same ancient word, not a loan.