Mac Dictionary defines it as:
an area of relatively level high ground.
• figurative a state of little or no change following a period of activity or progress : the peace process had reached a plateau.
• [as adj. ] denoting a group of American Indian peoples of the plateau country of western Canada and the U.S., including the Nez Percé.
verb ( -teaus, -teaued, -teauing) [ intrans. ]
reach a state of little or no change after a time of activity or progress : the industry’s problems have plateaued out.
ORIGIN late 18th cent.: from French, from Old French platel, diminutive of plat ‘level.’