CW-complex, Hausdorff space, second-countable space, sober space
connected space, locally connected space, contractible space, locally contractible space
Given a topological space (or locale) $X$, a subspace of $X$ is dense if its closure is all of $X$.
In locale theory, we have the curious property that any intersection of dense subspaces is still dense. (This of course fails rather badly for topological spaces, where the intersection of all dense topological subspaces is the space of isolated point?s.) One consequence is that every locale has a smallest dense sublocale, the double negation sublocale.
In point-set topology, a space is separable if and only if it has a dense subspace with countably many points.