Why do we call it a ramp?
Ramp refers to the area in airport where the aircraft are parked. This term comes from the days of seaplanes. From this site:


  The term ‘Ramp‘ traces its roots back to the days of seaplanes when there literally was a ramp from the water to the terminal parking area. 


In case of seaplanes, the area is actually an inclined plane between the shore and water. This is similar to the term boat ramp.

Note that this is used mainly in US. The ICAO (and FAA) terminology for the area is Apron.