Closure
From Conservapedia
In topology, the closure of a set 
is the intersection of all closed sets containing . Equivalently, the closure of is the union of and all limit points of
A closure operator is an abstract (category theory) form of the topological notion of closure which can be applied to any set . It is a function 
from to the power set of satisfying the following conditions:
-
(augmentation) - If
then 
(monotonicity) -
(idempotence)
In the category of topological spaces, this operator is isomorphic to the standard topological one.
