"Suborbital" is a term you'll be hearing a lot as Sir Richard Branson flies aboard Virgin Galactic's VSS Unity winged spaceship and Jeff Bezos flies aboard Blue Origin's New Shepard vehicle to touch the boundary of space and experience a few minutes of weightlessness.

But what exactly is "suborbital?" Simply put, it means that while these vehicles will cross the ill-defined boundary of space, they will not be going fast enough to stay in space once they get there.

If a spacecraft—or anything else, for that matter—reaches a speed of 17,500 mph (28,000 km/h) or more, instead of falling back to the ground, it will continuously fall around the Earth. That continuous falling is what it means to be in orbit and is how satellites and the Moon stay above Earth.

Swashbuckling entrepreneur Richard Branson hurtled into space aboard his own winged rocket ship Sunday in his boldest adventure yet, beating out fellow billionaire Jeff Bezos.

The nearly 71-year-old Branson and five crewmates from his Virgin Galactic space tourism company reached an altitude of about 53 miles (88 kilometers) over the New Mexico desert—enough to experience three to four minutes of weightlessness and see the curvature of the Earth—and then safely glided home to a runway landing.