pla·teau

/ˈplæt.əʊ/ /plætˈoʊ/

noun ¹

a large flat area of land that is high above sea level

noun ²

a state of little or no change following a period of activity or progress.

"the peace process had reached a plateau"

verb

reach a state of little or no change after a time of activity or progress.

"the industry's problems have plateaued out"