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.

Design Principles: Visual Weight And Direction
Smashing Magazine
Working with Visual Weight in Your Designs
Six Revisions
Visual Weight
Zurb University
Visual Balance and Weight Allocation for Usability
UX Magazine
Visual Weight of Primary and Secondary Action Buttons
UX Movement