billion vs milliard

billion and milliard both are nouns.

Word	Noun	Adjective	Verb	Adverb
billion	Yes	Yes	No	No
milliard	Yes	No	No	No

As nouns, milliard is a hyponym of billion; that is, milliard is a word with a more specific, narrower meaning than billion:

billion (noun)	milliard (noun)
the number that is represented as a one followed by 9 zeros	a billion
the number that is represented as a one followed by 12 zeros; in the United Kingdom the usage followed in the United States is frequently seen
a very large indefinite number (usually hyperbole)

billion (adjective)	milliard (adjective)
denoting a quantity consisting of one thousand million items or units in the United States
denoting a quantity consisting of one million million items or units in Great Britain