million vs zillion

million and zillion both are nouns.

Word	Noun	Adjective	Verb	Adverb
million	Yes	Yes	No	No
zillion	Yes	Yes	No	No

As nouns, million and zillion are synonyms defined as:

Other synonyms of million include bazillion, billion, gazillion, jillion, trillion.

million (noun)	zillion (noun)
the number that is represented as a one followed by 6 zeros	a very large indefinite number (usually hyperbole)
a very large indefinite number (usually hyperbole)

million (adjective)	zillion (adjective)
(in Roman numerals, M written with a macron over it) denoting a quantity consisting of 1,000,000 items or units	very large indeterminate number