-illion is the suffix used to identify large numbers, derived from million. There are a number of systems, including:
Short scale
In the English language, Brazil, Puerto Rico, Russia, Turkey and Greece, the most common system for -illion is called the short scale. In this scale, the suffix is added to a Latin-derived numerical prefix meaning n to indicate 10^(3n+3). For example, a quadrillion is 10^(3*4+3), or 10^(12+3), or 10^15. Because of the popularity of scientific notation, the illions above trillion are not commonly used practically.
Long scale
In the remainder of the world, the long scale system is prevalent. In this, the -illion suffix is added to the Latin prefix meaning n to indicate 10^(6n). Numbers of the form 10^(6n+3) can still be read as the word "thousand" in the respective language followed by n-illion.
Rowlett
The Rowlett system is a proposed system for numbers that uses Greek-derived numerical prefixes for n = 4 upward, denoting 10^(3n). 10^9 in this system is gillion. "Million" is common to both systems.
Indefinite words
The English language also includes a few -illion words that don't refer to any specified number; they are just exaggerations. These include zillion, jillion, and gazillion.