(noun) - Symmetry is what occurs when one side of something balances out or mirrors the other. If you were to draw a midline from the top of your forehead, down the middle of your nose, over your lips and to the bottom of your chin, you could see that your eyes, ears, nostrils, and teeth all mirror each other on either side of the line. That's symmetry.
In a piece of art, symmetry is present in much the same way when the elements within balance each other. An excellent example of a symmetrical piece is Leonardo's The Last Supper.
symmetrical balance, formal balance
"Rhythm, symmetry, and a happy combination of elegance and utility - a blend often desired in later days of hope and struggle - these have been fully attained, and with them a delight in quiet communion with Nature, expressing as she does the sense of beauty in orderliness." - Marie Luise Gothein, History of Garden Art, 1928