I read this somewhere; I forget where and perhaps the actual details are different, but here's a definition of eternity that I like:
Imagine a block of granite a thousand miles thick, floating in space. Every thousand years, a raven alights on it and sharpens its beak with two flicks...back and forth. The raven flies off, not to return for another millennium. That raven will wear eventually wear completely through that thousand-mile chunk of granite, and when it does, one second of eternity will have ticked by.
Of course, this definition isn't accurate as it implies an ending to eternity. But it's a way for me to at least start to wrap my mind around what is essentially an incomprehensible concept.
An alternate definition for local residents involves the traffic signal on Holiday Hill Road at the intersection with Loop 250.