In mathematics, a **pseudometric space** is a generalization of a metric space in which the distance between two distinct points can be zero. Pseudometric spaces were introduced by Đuro Kurepa^{[1]}^{[2]} in 1934. In the same way as every normed space is a metric space, every seminormed space is a pseudometric space. Because of this analogy, the term semimetric space (which has a different meaning in topology) is sometimes used as a synonym, especially in functional analysis.

When a topology is generated using a family of pseudometrics, the space is called a gauge space.

A pseudometric space is a set together with a non-negative real-valued function called a **pseudometric**, such that for every

*Symmetry*:*Subadditivity*/*Triangle inequality*:

Unlike a metric space, points in a pseudometric space need not be distinguishable; that is, one may have for distinct values

Any metric space is a pseudometric space. Pseudometrics arise naturally in functional analysis. Consider the space of real-valued functions together with a special point This point then induces a pseudometric on the space of functions, given by

for

A seminorm induces the pseudometric . This is a convex function of an affine function of (in particular, a translation), and therefore convex in . (Likewise for .)

Conversely, a homogeneous, translation-invariant pseudometric induces a seminorm.

Pseudometrics also arise in the theory of hyperbolic complex manifolds: see Kobayashi metric.

Every measure space can be viewed as a complete pseudometric space by defining

for all where the triangle denotes symmetric difference.

If is a function and *d*_{2} is a pseudometric on *X*_{2}, then gives a pseudometric on *X*_{1}. If *d*_{2} is a metric and *f* is injective, then *d*_{1} is a metric.

The **pseudometric topology** is the topology generated by the open balls

which form a basis for the topology.

The difference between pseudometrics and metrics is entirely topological. That is, a pseudometric is a metric if and only if the topology it generates is T_{0} (that is, distinct points are topologically distinguishable).

The definitions of Cauchy sequences and metric completion for metric spaces carry over to pseudometric spaces unchanged.^{[5]}

The vanishing of the pseudometric induces an equivalence relation, called the **metric identification**, that converts the pseudometric space into a full-fledged metric space. This is done by defining if . Let be the quotient space of by this equivalence relation and define

This is well defined because for any we have that and so and vice versa. Then is a metric on and is a well-defined metric space, called the

The metric identification preserves the induced topologies. That is, a subset is open (or closed) in if and only if is open (or closed) in and is saturated. The topological identification is the Kolmogorov quotient.

An example of this construction is the completion of a metric space by its Cauchy sequences.