some definitions related to open sets 2 (isolated and limit points)

Given a metric space $\Omega$ , a open set $A\subset \Omega$  and a member $a\in A$ .

We say a point $a$  is isolated, if, for any $\delta>0$ , a intersection of the open ball
$B(a,\delta )$  and $A$  is $a$ . It means there is not any point of $A$  except $a$ 
in a neighborhood of $a$ .

If any neighborhoods of $a$  has infinite points of $A$ , $a$ is called a accumulation point
or a limit point.

These will be understood easily. However, these are important definitions in the topology.
Please try to assume relations of various kinds of a set.