What counts as international research collaboration? The gap between conceptualisation and operationalisation in higher education research evaluation

International research collaboration (IRC) is now a central feature of higher education systems and is increasingly used as evidence of institutional quality, research excellence, and internationalisation. IRC is usually evaluated through co-authorships, especially international co-authored publications identified through author affiliation data. Literature has typically analysed IRC through bibliometric mappings of countries, institutions, disciplines, and researchers, or by examining the determinants and effects of collaboration. Less attention has been given to the concept-operation gap: whether the indicators used to evaluate IRC adequately correspond to the broader meanings attached to collaboration in higher education policy, research evaluation, and institutional strategy. This study examines that gap through a systematic review of 723 studies from Scopus, Web of Science, and ProQuest Social Science. It combines bibliometric mapping of the full corpus with close content analysis of a purposive subset of 94 studies and covers Chinese-language as well as English-language publications. The findings show that IRC is conceptually plural but operationally compressed. It is conceptualised as a process, relationship, orientation, output, and policy instrument, but operationalised predominantly through publication-visible indicators. Across the corpus, 97.2% of studies incorporate at least one bibliometric element, most commonly international co-authorship inferred from affiliation data. This concentration privileges formal, indexable, and publication-visible collaboration, while making capacity building, co-supervision, mentoring, informal intellectual exchange, and long-term partnership development harder to recognise. The study is novel in showing how IRC becomes a metricised governance object and calls for more reflexive approaches to evaluating international collaboration in higher education.