From the Dictionary of Obscure Sorrows, one definition less hopeful:
n. a state of exhaustion inspired by acts of senseless violence, which force you to revise your image of what can happen in this world—mending the fences of your expectations, weeding out all unwelcome and invasive truths, cultivating the perennial good that’s buried under the surface, and propping yourself up like an old scarecrow, who’s bursting at the seams but powerless to do anything but stand there and watch.
and another a little more hopeful:
n. the glint of goodness inside people, which you can only find by sloshing them back and forth in your mind until everything dark and gray and common falls away, leaving behind a constellation at the bottom of the pan—a rare element trapped in exposed bedrock, washed there by a storm somewhere upstream.
and a third one, full of recognition and truth (surely the most famous, and for good reason):
n. the realization that each random passerby is living a life as vivid and complex as your own—populated with their own ambitions, friends, routines, worries and inherited craziness—an epic story that continues invisibly around you like an anthill sprawling deep underground, with elaborate passageways to thousands of other lives that you’ll never know existed, in which you might appear only once, as an extra sipping coffee in the background, as a blur of traffic passing on the highway, as a lighted window at dusk.
and my favourite:
n. the desire to care less about things—to loosen your grip on your life, to stop glancing behind you every few steps, afraid that someone will snatch it from you before you reach the end zone—rather to hold your life loosely and playfully, like a volleyball, keeping it in the air, with only quick fleeting interventions, bouncing freely in the hands of trusted friends, always in play.