Blind eye
A blind eye of a chain C is a term proposed by Patrick Traill for use in the context of Benson's Definition of Unconditional Life for an enclosed region that fails to be vital for C (an eye of C) because not all vacant points in it are adjacent to the C.
Benson's theorem states that a set S of chains is pass-alive (cannot be capture by the opponent however many moves in a row he has) if and only if each chain in S has two eyes enclosed by S; an eye of a chain is a maximal connected collection of empty points and opposing stones such that every vacant point (and, if suicide is allowed, every opposing stone) in it is adjacent to C.