topological invariant

A topological invariant of a space $X$ is a property that depends only on the topology of the space, i.e. it is shared by any topological space homeomorphic to $X$. Common examples include compactness (http://planetmath.org/Compact), connectedness (http://planetmath.org/ConnectedSpace), Hausdorffness (http://planetmath.org/T2Space), Euler characteristic, orientability (http://planetmath.org/Orientation2), dimension (http://planetmath.org/InvarianceOfDimension), and like homology, homotopy groups, and K-theory.

Properties of a space depending on an extra structure such as a metric (i.e. volume, curvature, symplectic invariants) typically are not topological invariants, though sometimes there are useful interpretations of topological invariants which seem to depend on extra information like a metric (for example, the Gauss-Bonnet theorem).