density vs spacing

density and spacing both are nouns.

Word	Noun	Adjective	Verb	Adverb
density	Yes	No	No	No
spacing	Yes	No	No	No

As nouns, spacing is a hypernym of density; that is, spacing is a word with a broader meaning than density:

density: the spatial property of being crowded together
spacing: the property possessed by an array of things that have space between them

Other hypernyms of density include spatial arrangement.

density (noun)	spacing (noun)
the amount per unit size	the property possessed by an array of things that have space between them
the spatial property of being crowded together	the time between occurrences of a repeating event