Balance
Distribution of visual weight across a composition that reads as stable, whether achieved through symmetry or through offsetting asymmetry.
In practice
Balance is the sense that a composition's visual weight is spread so nothing seems about to tip. Weight comes from size, darkness, color intensity, texture and isolation. A symmetric book cover balances by mirroring; an asymmetric poster balances a big pale shape against a small dark one. Designers often check balance by squinting or flipping the design horizontally, which reveals heavy corners they had stopped noticing.
Often confused with Symmetry
Symmetry is one way to achieve balance, by mirroring; balance can also come from unequal elements arranged asymmetrically.
Try this
Flip one of your layouts horizontally and note whether any area suddenly looks heavier than before.