The Spanish word ciento, which gets shortened to cien before a noun, means ‘a hundred.’ The synonymous Latin original was centum, whose cent- corresponds to the hund- in the native English cognate hundred (with the -red developing from a Germanic root that meant ‘reckoning, number,’ senses similar to those of the apparent Latin cognate ratio).
According to the American Heritage Dictionary, Latin centum, which was pronounced kentum, had developed from Indo-European *dkm-tom, whose first element led to Latin decem and Spanish diez as well as English ten. It’s not clear what sense the Indo-European suffix -tom conveyed in its own right, but the compound *dkm-tom ultimately came to mean ‘ten groups of ten.’§ As for form, the d of *dkm-tom was eventually lost, and the -kmt- of the remainder went on to produce Latin cent(um) and English hund(red).
The Modern Latin phrase per centum ‘for [each] hundred’ has become Spanish por ciento. English originally borrowed the Latin phrase in full, then began abbreviating it per cent., with a period to show that cent. was indeed an abbreviation. Only in the early part of the 20th century—another derivative, like Spanish centuria, of Latin centum—did English drop the period, writing at first per cent, then the combined percent that is the usual current form.
The French descendant of Latin centum is cent, which Americans have adopted as a monetary unit worth one one-hundredth of a dollar. The analog in Spanish-speaking countries is the centavo, or in some countries the centésimo.
§ On that score, I have to report that one day in the early 1970s I was in a supermarket on Long Island and overheard a nearby woman tell her daughter that ten times ten is a hundred, which is true enough, and that a hundred times a hundred is a thousand, which is not. Etymology could have ridden to the rescue there, because Germanic *thūs-hundi‑, the ancestor of English thousand, meant ‘a swollen hundred,’ which is to say ‘ten times a hundred.’
© 2015 Steven Schwartzman