In

topology, a point

**p** is said to be an

accumulation point of a

set **X** if every

neighborhood of

**p** contains points of

**X** distinct from

**p**. (Note that

**p** need not be in

**X**.)

For example, on the real line with the usual topology, 0 is an accumulation point of the open interval bounded by 0 and 1, because any neighborhood of 0 contains points (infinitely many, in fact) lying between 0 and 1.