According to Wikipedia, the non-romantic, full-on nerd definition of a light cone is

In special relativity, a light cone (or null cone) is the surface describing the temporal evolution of a flash of light in Minkowski spacetime. This can be visualized in 3-space if the two horizontal axes are chosen to be spatial dimensions, while the vertical axis is time.

More about light cones here.

There are more romantic ways to put it, however. Paraphrasing to Matt Webb, the author of Mind Hacks and creator of the very cool Light Cone RSS feed says

From the moment of your birth lightwaves that have bounced off your body have been expanding around the Earth into space in a sphere who’s surface (not surprisingly) is growing at the speed of light. Meanwhile, light coming from objects inside that same growing sphere have been reaching you. Put the two together and you’ve got your light sphere.

He said it here.

Webb’s Light Cone page only goes back 50 years but the general principle is pretty cool. Lightwaves, even those reflecting sources far more feeble than a candle, may be about as small as stable particles of matter get but they’re exceedingly long-lived. Lightwaves from the moment of your birth or mine (or anyone else’s who might have been born, no matter how long ago, on on, say, December 25th) are still out there, moving away at 186,000-odd miles per second. Some may travel on till the end of time. Always something to think about.

Happy HNT (or Half-nekkid Thursday!)