Hi amirkabir,
Georgetown is certainly asking a wide open question. It's almost as open-ended as the grandaddy of all open-ended questions, Stanford's "What is most important to you and why?" I can completely understand your confusion about what they are looking for, as it's very hard to figure out one's own "essence."
The first hint that they are looking for something touchy-feely is that they are looking for you to describe the core of your being personally. At least that's how I see the word essence, which is defined as
1.The intrinsic or indispensable properties that serve to characterize or identify something.
2. The most important ingredient; the crucial element.
I like the second definition, which is your most important ingredient. So you have to ask yourself, how do you want to characterize yourself? You might want to characterize yourself as someone who is incredibly curious, or hardworking, or empathetic, or all three. You might give examples where you show those characteristics. I recommend you tell a number of stories that show the three-dimensional person you are.
You might also want to talk about the stuff that drives you. What makes you get up in the morning? What excites you? What are the themes that keep showing up over and over? If the essence of who you are is someone who likes to connect to and motivate people, it's going to show up at work, during your undergraduate days, in your extra-curricular activities, and with your close friends.
I know it's hard, but there are no wrong answers. Give yourself the freedom to describe yourself in a creative way. You may just discover something new about yourself in the process!
Good luck with this!