Real Dictionary

plateau
countable noun: A plateau is a large area of high and fairly flat land.
countable noun: If you say that an activity or process has reached a plateau, you mean that it has reached a stage where there is no further change or development.
verb: If something such as an activity, process, or cost plateaus or plateaus out, it reaches a stage where there is no further change or development.

plateau in American English
an elevated tract of more or less level land; tableland; mesa
to become relatively stable or constant, as in position
intransitive verb: to reach a state or level of little or no growth or decline, esp. to stop increasing or progressing; remain at a stable level of achievement; level off
noun: a land area having a relatively level surface considerably raised above adjoining land on at least one side, and often cut by deep canyons
transitive verb: to cause to remain at a stable level, esp. to prevent from rising or progressing

plateau in British English
to remain at a stable level for a relatively long period
noun: a state of central Nigeria, formed in 1976 from part of Benue-Plateau State: tin mining. Capital: Jos. Pop: 3 178 712 (2006). Area: 30 913 sq km (11 936 sq miles)
noun: a wide mainly level area of elevated land