compactness - the consistency of a compact solid

relative density - the ratio of the density of something to the density of a standard

low density, rarity, tenuity - a rarified quality; "the tenuity of the upper atmosphere"

spatial arrangement, spacing - the property possessed by an array of things that have space between them

bits per inch, bpi - a measure of how densely information is packed on a storage medium

flux density, flux - (physics) the number of flux changes per unit area