Period
In place value, a period is a group of three digits separated by commas, such as the ones period, thousands period, and millions period.
Definition
In large numbers, a period is a group of three digits separated by commas, grouped right to left; each group has its own name: ones, thousands, millions, billions, and so on, with each period representing a $1{,}000$-fold increase over the previous one. Reading numbers by period simplifies naming large numerals, and this grouping by three reflects that $1{,}000 = 10^3$ is the multiplier between period names, aligning with SI prefixes in scientific and engineering notation (kilo $= 10^3$, mega $= 10^6$, giga $= 10^9$).
Example
In $4{,}752{,}391$: the ones period is $391$, the thousands period is $752$, and the millions period is $4$, read as "four million, seven hundred fifty-two thousand, three hundred ninety-one." $2{,}047{,}000{,}816$ has four periods: $2$ (billions), $047$ (millions), $000$ (thousands), $816$ (ones). The short scale (US/UK modern) sets $10^9 =$ billion, while the long scale (older European) sets $10^9 =$ milliard and $10^{12} =$ billion, showing the period structure stays the same even as naming conventions differ.
Key Insight
Commas mark the boundaries between periods and make large numbers much easier to read at a glance. The choice of period size three, rather than four or two, is historically linked to the adoption of Hindu-Arabic numerals in Europe, where grouping by thousands was a natural fit for the existing vocabulary of large number names.