(noun) - As a basic principle of art (specifically of design), balance refers to the ways in which the elements (lines, shapes, colors, textures, etc.) of a piece are arranged. Balance is one of those useful terms to know, if one is to employ Art Speak.
Balance can be symmetrical ("formal"), where elements are given equal "weight" from an imaginary line in the middle of a piece. For the most basic example of symmetry, think of your eyes in relation to either side of your nose.
Balance doesn't necessarily mean symmetry, though. Asymmetrical ("informal") balance occurs when elements are placed unevenly in a piece, but work together to produce harmony overall.