Four, it follows an established set of parameters and rules, and requires some artificial boundary of time and space.

NEWYORKER: Master of Play

Such a space has no boundary, even though it has a finite volume.

ECONOMIST: The shape of the universe

"If the Wright brothers were starting out today they'd never get off the ground, " fumes Ky Michaelson, who spent two years dickering with bureaucrats at an offshoot of the FAA to get a license to launch to 390, 000 feet, well past the boundary zone between the Earth's atmosphere and space.

FORBES: "Let's Punch A Hole In the Sky"

But we are the only species to have defined a geological period by our activity something usually performed by major glaciations, mass extinction and the colossal impact of objects from outer space, like the one that defines the upper boundary of the Cretaceous.

FORBES: On The Anthropocene Age