plateau
noun
[ ˈplatəʊ ]
• an area of fairly level high ground.
• a state of little or no change following a period of activity or progress.
• "the peace process had reached a plateau"
plateau
verb
• reach a state of little or no change after a period of activity or progress.
• "the industry's problems have plateaued out"
Origin:
late 18th century: from French, from Old French platel, diminutive of plat ‘level’.