orbital velocity:  Bodies in space move around their orbits at different speeds.  The speed at which a given body orbits is its orbital velocity.  Orbital velocity is also used to describe the speed needed to place a satellite in orbit.

ozone layer:  At the upper level of the stratosphere, oxygen atoms combine into a molecule that is rarely found at Earth's surface.  This rare form of oxygen molecule is called ozone.  The ozone oxygen molecule has three atoms instead of the usual two.  At the top of the stratosphere (a layer of Earth's atmosphere) is a thin region where this ozone molecule is found in great quantities.  This thin region is called the ozone layer.  The ozone layer is about 15 miles (24 km) above Earth's surface.   Ozone absorbs the Sun's ultraviolet radiation.  The ozone layer prevents harmful radiation from reaching Earth's surface.