In mathematics, a limit point, accumulation point, or cluster point of a set S {\displaystyle S} in a topological space X {\displaystyle X} is a point x {\displaystyle x} that can be "approximated" by points of S {\displaystyle S} in the sense that every neighbourhood of x {\displaystyle x} with respect to the topology on X {\displaystyle X} also contains a point of S {\displaystyle S} other than x {\displaystyle x} itself. Wikipedia