astronomy unit vs AU

astronomy unit and AU both are nouns.

Word	Noun	Adjective	Verb	Adverb
astronomy unit	Yes	No	No	No
AU	Yes	No	No	No

As nouns, AU is a hyponym of astronomy unit; that is, AU is a word with a more specific, narrower meaning than astronomy unit:

astronomy unit: a linear unit used for astronomical distances
AU: a unit of length used for distances within the solar system; equal to the mean distance between the Earth and the Sun (approximately 93 million miles or 150 million kilometers)

Other hyponyms of astronomy unit include light-year, light year, light hour, light minute, light second, Astronomical Unit, parsec, secpar.

astronomy unit (noun)	AU (noun)
a linear unit used for astronomical distances	a unit of length used for distances within the solar system; equal to the mean distance between the Earth and the Sun (approximately 93 million miles or 150 million kilometers)