Visual Weight.

The notion that design elements have varied weights; that is, some objects, even on a two-dimensional medium, can appear to be heavier than others. Visual weight is a powerful concept that allows us to create visual hierarchy, symmetry, balance, and harmony in designs.

Working with Visual Weight in Your DesignsWebFX
Visual WeightZurb
Design Principles: Visual Weight And DirectionSmashing MagazineDecember 12th, 2014
Visual Balance and Weight Allocation for UsabilityUX MagazineMarch 14th, 2013
Visual Weight of Primary and Secondary Action ButtonsUX MovementJuly 7th, 2011