  Metric graphs are often introduced based on combinatorics, upon "associating"
each edge of a graph with an interval; or else, casually "gluing" a collection
of intervals at their endpoints in a network-like fashion. Here we propose an
abstract, self-contained definition of metric graph. Being mostly topological,
it doesn't require any knowledge from graph theory and already determines
uniquely several concepts that are commonly and unnecessarily \textit{defined}
in the literature. Nevertheless, many ideas mentioned here are folklore in the
quantum graph community: we discuss them for later reference.
