Brett, just as gmik thought. The term zillion is considered slang.
Indefinite and fictitious large numbers
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By analogy to the names billion, trillion, and so forth for large powers of ten, words such as zillion and bazillion are often used as fictitious names for an unspecified, large number of indefinite size, or as part of a large, indefinite measurement. However, it is often used in children's literature. These words are often used in hyperbole. The size is dependent upon the context to which it is used, but can typically be considered large enough to be unfathomable by the average human mind.
Many similar words are used, such as jillion, ananillion, bajillion, squillion, skillion, gonillion, kabillion, kajillion, gajillion, umptillion, gagillion, gadzillion, godzillion, quillion, julillion, hojillion, and grillion.
Typically, these words are used in a humorous context, or in loose, unconfined conversation to present an un-guessably large number. It may also be used to impress someone with the concept of an ambiguous numerical enormousness.