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.