diameter vs calibre

diameter and calibre both are nouns.

Word	Noun	Adjective	Verb	Adverb
diameter	Yes	No	No	No
calibre	Yes	No	No	No

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

diameter: the length of a straight line passing through the center of a circle and connecting two points on the circumference
calibre: diameter of a tube or gun barrel

Other hyponyms of diameter include bore, caliber, gauge, windage.

diameter (noun)	calibre (noun)
the length of a straight line passing through the center of a circle and connecting two points on the circumference	a degree or grade of excellence or worth
a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)	diameter of a tube or gun barrel