In geometry, a set S {\displaystyle S} in the Euclidean space R n {\displaystyle \mathbb {R} ^{n}} is called a star domain if there exists an s 0 ∈ S {\displaystyle s_{0}\in S} such that for all s ∈ S, {\displaystyle s\in S,} the line segment from s 0 {\displaystyle s_{0}} to s {\displaystyle s} lies in S. Wikipedia