In topology and related fields of mathematics, a set *U* is called **open** if, intuitively speaking, you can "wiggle" or "change" any point *x* in *U* by a small amount in any direction and still be inside *U*.
In other words, *x* can't be on the edge of *U*.

As a typical example, consider the open interval (0,1) consisting of all real numbers *x* with 0 < *x* < 1.
If you "wiggle" such an *x* a little bit (but not too much), then the wiggled version will still be a number between 0 and 1.
Therefore, the interval (0,1) is open.
However, the interval (0,1] consisting of all numbers *x* with 0 < *x* ≤ 1 is not open; if you take *x* = 1 and wiggle a tiny bit in the positive direction, you will be outside of (0,1].

Note that whether a given set *U* is open depends on the surrounding space, the "wiggle room".
For instance, the set of rational numbers between 0 and 1 (exclusive) is open *in the rational numbers*, but it is not open *in the real numbers*.
Note also that "open" is not the opposite of "closed".
First, there are sets which are both open and closed (called *clopen sets*); in **R** and other connected spaces, only the empty set and the whole space are clopen, while the set of all rational numbers smaller than √2 is clopen in the rationals.
Also, there are sets which are neither open nor closed, such as (0,1] in **R**.

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The concept of open sets can be formalized in various degrees of generality.

A subset *U* of Euclidean *n*-space **R**^{n} is called *open* if, given any point *x* in *U*, there exists a real number ε > 0 such that, given any point *y* in **R**^{n} whose Euclidean distance from *x* is smaller than ε, *y* also belongs to *U*.

Intuitively, ε measures the size of the allowed "wiggles".

A subset *U* of a metric space (*M*,*d*) is called *open* if, given any point *x* in *U*, there exists a real number ε > 0 such that, given any point *y* in *M* with *d*(*x*,*y*) < ε, *y* also belongs to *U*.

This generalizes the Euclidean space example, since Euclidean space with the Euclidean distance is a metric space.

In topological spaces, the concept of openness is taken to be fundamental.
One starts with an arbitrary set *X* and a family of subsets of *X* satisfying certain properties that every "reasonable" notion of openness is supposed to have.
(Specifically: the union of open sets is open, the finite intersection of open sets is open, and in particular the empty set and *X* itself are open.)
Such a family **T** of subsets is called a *topology* on *X*, and the members of the family are called the *open sets* of the topological space (*X*,**T**).

This generalises the metric space definition: If you start with a metric space and define open sets as before, then the family of all open sets will form a topology on the metric space. Every metric space is hence in a natural way a topological space. (There are however topological spaces which are not metric spaces.)

Every subset *A* of a topological space *X* contains a (possibly empty) open set; the largest such open set is called the interior of *A*.
It can be constructed by taking the union of all the open sets contained in *A*.

Given topological spaces *X* and *Y*, a function *f* from *X* to *Y* is *continuous* if the preimage of every open set in *Y* is open in *X*.
The map *f* is called *open* if the image of every open set in *X* is open in *Y*.

A manifold is called **open** if it is a manifold without boundary and if it is not compact.
This notion differs somewhat from the openness discussed above.