pla·teau 
(plă-tō)
(plă-tō)n. pl. pla·teaus or pla·teaux (-tōz)
1. An elevated, comparatively level expanse of land; a tableland.
2. A relatively stable level, period, or state: Mortgage rates declined, then reached a plateau.
intr.v. pla·teaued, pla·teau·ing, pla·teaus
To reach a stable level; level off: "The tension seemed to grow by degrees, then it plateaued" (Tom Clancy).
[French, from Old French platel, platter, from plat, flat; see PLATE.]