  The term Open Access not only describes a certain model of scholarly
publishing -- namely in digital format freely accessible to readers -- but
often also implies that free availability of research results is desirable, and
hence has a normative character. Together with the large variety of presently
used definitions of different Open Access types, this normativity hinders a
systematic investigation of the development of open availability of scholarly
literature. In this paper, we propose a non-normative definition of Open Access
and its usage as a neutral, descriptive term in bibliometric studies and
research on science. To this end, we first specify what normative figures are
commonly associated with the term Open Access and then develop a neutral
definition. We further identify distinguishing characteristics of openly
accessible literature, called dimensions, and derive a classification scheme
into Open Access categories based on these dimensions. Additionally, we present
an operationalisation method to assign scientific publications to the
respective categories in practice. Here, we describe useful data sources, which
can be employed to gather the information needed for the classification of
scholarly works according to the presented classification scheme.
