The relative newness of 'impact' as a criterion for research assessment has meant that there is yet to be an empirical study examining the process of its evaluation. This article is part of a broader study which is exploring the panel-based peer and end-user review process for societal impact evaluation using the UK's national research assessment exercise, the Research Excellence Framework (REF) 2014, as a case study. In particular, this article explores the different perceptions REF2014 evaluators had regarding societal impact, preceding their evaluation of this measure as part of REF2014. Data are drawn from 62 interviews with evaluators from the health-related Panel A and its subpanels, prior to the REF2014 exercise taking place. We show how going into the REF exercise, evaluators from Panel A had different perceptions about how to characterize impact and how to define impact realization in terms of research outcomes and the research process. We conclude by discussing the implications of our findings for future impact evaluation frameworks, as well as postulating a series of hypotheses about the ways in which evaluators' different perceptions going into an impact assessment could potentially influence the evaluation of impact submissions. Using REF2014 as a case study, these hypotheses will be tested in interviews with REF2014 evaluators post-assessment.