I. plat ‧ eau 1 /ˈplætəʊ $ plæˈtoʊ/ BrE AmE noun ( plural plateaus or plateaux /-təʊz $ -ˈtoʊz/) [countable]
[ Date: 1700-1800 ; Language: French ; Origin: Old French platel 'flat piece of metal' , from plat ; ⇨ ↑ plate 1 ]
1 . a large area of flat land that is higher than the land around it
2 . a period during which the level of something does not change, especially after a period when it was increasing:
Inflation rates have reached a plateau.
II. plateau 2 BrE AmE verb [intransitive]
if something plateaus, it reaches and then stays at a particular level:
The athletic footwear market has not yet plateaued.