A suborbital flight path, taken by a space launch vehicle, guided missile, or other vehicle, flies high enough to enter outer space, but, for one of several reasons, does not achieve orbit. It may not have sufficient energy to overcome gravity, or its trajectory, after its engine cuts off, will intersect the earth rather than fly high enough to enter orbit.
Scientific vehicles that intentionally do not reach orbit, especially those launched to observe the upper atmosphere, are often called sounding rockets. Some suborbital flights are intended to test vehicles that are planned to go into orbit on later flights. There have also been suborbital flights that fired additional rockets as they curved downward, so they could simulate the atmospheric reentry of a orbital or deep space probe entering atmosphere at especially high speed.
Generally, the vehicle must reach at least 62.5mi/100 km to be considered suborbital rather than an upper atmospheric probe.