spectral set

spectral set (plural spectral sets)

1. (mathematics) In operator theory, a set ${\displaystyle X\subseteq \mathbb {C} }$ such that, for a (possibly unbounded) linear operator ${\displaystyle T}$ on a Banach space, the spectrum of ${\displaystyle T}$ is in ${\displaystyle X}$ and von Neumann's inequality holds for ${\displaystyle T}$ on ${\displaystyle X}$.