Let $(X,\mathcal{T})$ be a \reftext{def:topological-space-2026a}{topological space}. We say that $X$ is second countable if there exists a basis $\mathcal{B}$ for $\mathcal{T}$ in the sense of \ref{def:basis-topology-2026a} such that $\mathcal{B}$ is countable.