A plateau of a function is a region where a function has constant value.

More formally, let $U$ and $V$ be topological spaces. A plateau for a scalar function $f: U \to V$ is a path-connected set of points $P \subseteq U$ such that for some $y$ we have

\begin{equation} \forall p \in P, f(p) = y \end{equation} Please take note that this entry is not authoritative. If you know of a more standard definition of ``plateau'', please contribute it, thank you.