How much is packed into a given space. Density decides how much attention a surface asks for.
Latin densitas (thickness, density), from densus (dense, thick, crowded); how much is packed into a given space, deciding by that packing how much attention the space will demand.
Latin densus is crowded, and density was first a word about closeness, not about weight — how near things stand to each other inside a fixed space. A crowded surface asks more of the eye than a sparse one holding the same amount.