Per Centum -

Parts per 10,000 (one-hundredth of 1 percent). Per Cent Mille (pcm): Parts per 100,000 (1 in 100,000).

The core, simple formula for a percentage is per centum

If a whole is divided into 100 parts, a "percentage" indicates how many of those parts make up the subset being discussed. Parts per 10,000 (one-hundredth of 1 percent)

The term evolved over time. "Per" was often abbreviated to "p", and "cento" was contracted until it eventually became the modern symbol: %. Parts per 10

The phrase emerged from per centum (16th century), and its usage was heavily refined by Italian merchants, who frequently used "per cento" for calculating taxes, interest, and discounts.