In a metric space $M$ a set $O$ is called an open subset of $M$ or just open, if for every $x\in O$ there is an open ball $S$ around $x$ such that $S\subset O$. If $d(x,y)$ is the distance from $x$ to $y$ then the open ball $B_{r}$ with radius $r>0$ around $x$ is given as:

Using the idea of an open ball one can define a neighborhood of a point $x$. A set containing $x$ is called a neighborhood of $x$ if there is an open ball around $x$ which is a subset of the neighborhood.

These neighborhoods have some properties, which can be used to define a topological space using the Hausdorff axioms for neighborhoods, by which again an open set within a topological space can be defined. In this way we drop the metric and get the more general topological space. We can define a topological space $X$ with a set of neighborhoods of $x$ called $U_{x}$ for every $x\in X$, which satisfy

1. 1.

   $x\in U$ for every $U\in U_{x}$

2. 2.

   If $U\in U_{x}$ and $V\subset X$ and $U\subset V$ then $V\in U_{x}$ (every set containing a neighborhood of $x$ is a neighborhood of $x$ itself).

3. 3.

   If $U,V\in U_{x}$ then $U\cap V\in U_{x}$.

4. 4.

   For every $U\in U_{x}$ there is a $V\in U_{x}$, such that $V\subset U$ and $V\in U_{p}$ for every $p\in V$.

The last point leads us back to open sets, indeed a set $O$ is called open if it is a neighborhood of every of its points. Using the properties of these open sets we arrive at the usual definition of a topological space using open sets, which is equivalent to the above definition. In this definition we look at a set $X$ and a set of subsets of $X$, which we call open sets, called $\mathcal{O}$, having the following properties:

1. 1.

   $\emptyset\in\mathcal{O}$ and $X\in\mathcal{O}$.

2. 2.

   Any union of open sets is open.

3. 3.

   Finite intersections of open sets are open.

Note that a topological space is more general than a metric space, i.e. on every metric space a topology can be defined using the open sets from the metric, yet we cannot always define a metric on a topological space such that all open sets remain open.