To me personally, art is something which inspires some form of meaningful emotion in me, like awe, sadness, joy, discomfort, etc. Games allow for a unique opportunity to deliver art to an audience: because the medium is interactive, not only am I seeing the art, I am also participating in that art. (Some spoilers follow): In Portal, when I first went "behind the scenes" and saw what other test subjects had written on the wall, and in particular when I noticed that the writing looked suspiciously like it had been written in blood, I felt this sick feeling in the pit of my stomach, and for a brief moment, I was the one trapped in that test facility with a psychotic AI. The game in many subtle ways had drawn me in, and made me feel as if I was in that world.
To give another example: in the game Flower, the premise seemed simple enough: bloom all the flowers, everywhere. But the experience was far from simple: the way the music began each level soft and simple, and built in complexity as I continued, the way the six-axis controls forced me to physically move as I directed the wind, the dizzying, beautiful visuals; all of these things gave me the feeling that I myself was soaring on the wind, collecting an enormous trail of petals behind me, and I have not played any other games that have made me feel such an intense joy.
Anyway, sorry that got kind of long-winded; I hope at least some of it will help you with your essay. Good luck!