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}
   .