Is there a word for something that can be jumped off of? For example the ground can jumped off of, but you can not jump off something which is falling through the air. I'm looking for an adjective.
What's wrong with a jump off? True, a jump off is usually associated with a location, as in
The jump off point for the parachutists was ten miles south of Interstate 79, just outside Albuquerque, New Mexico.
I don't see why you can't use jump off in referring to an object from which you are jumping.
The skateboarders used a specially constructed fifteen-foot-high platform as a jump off for their most dangerous stunt.
To jump off of something, it must be higher than wherever you are jumping to or falling to after jumping. Therefore the word elevation can be used for something off which a person can jump. The ground alone, at least level ground as @ Medica observes, can be jumped from, but not off.
el·e·va·tion [el-uh-vey-shuhn] noun
3. an elevated place, thing, or part; an eminence. http://dictionary.reference.com/browse/elevation