  Quantifying the interdisciplinarity of a research is a relevant problem in
the evaluative bibliometrics. The concept of interdisciplinarity is ambiguous
and multidimensional. Thus, different measures of interdisciplinarity have been
propose in the literature. However, few studies have proposed interdisciplinary
metrics without previously defining classification sets, and no one use the
co-citation network for this purpose. In this study we propose an
interdisciplinary metric based on the co-citation network. This is a way to
define the publication's field without resorting to pre-defined classification
sets. We present a characterization of a publication's field and then we use
this definition to propose a new metric of the interdisciplinarity degree for
publications (papers) and journals as units of analysis. The proposed measure
has an aggregative property that makes it scalable from a paper individually to
a set of them (journal) without more than adding the numerators and
denominators in the proportions that define this new indicator. Moreover, the
aggregated value of two or more units is strictly among all the individual
values.
