Large numbers
Confusingly, there are two systems for naming numbers using the ending -illion. The history of which system has been used in which countries at which times is complex.
In both systems, a "million" means a thousand thousands or 106.
Above the "million," one system introduces a new name for each new group of three zeroes. In particular, a "billion" means a thousand millions. This is American usage and has been for a long time.
Another system, which can be called the "long scale," introduces a new name for each group of six zeroes. Thus, a billion means a million millions. This used to be British usage, but no longer is; newspapers and official government reports now use the same system as the U. S., and this system is the one now taught in British schools. Some older Britons remain attached to the older usage.
As a practical matter, it is important to be aware of the difference in meaning when reading old material, and to avoid using either set of names—especially the common but ambiguous billion—in any context where misunderstanding can occur.
In technical or scientific writing, the problem does not arise because it is customary to use scientific notation (1012) or SI prefixes (tera-).
| "Short scale" | "Long scale" | Math |
| thousand | thousand | 103 = 1,000 |
| million | million | 106 = 1,000,000 |
| billion | thousand million or milliard | 109 = 1,000,000,000 |
| trillion | billion | 1012 = 1,000,000,000 |
| quadrillion | thousand billion or (very rarely) billiard | 1015 = 1,000,000,000,000 |
| quintillion | trillion | 1018 = 1,000,000,000,000,000 |