Background Health care workers (HCWs) are susceptible to moral stress and distress when they are faced with morally challenging situations where it is difficult to act in line with their moral standards. In times of crisis, such as disasters and pandemics, morally challenging situations are more frequent, due to the increased imbalance between patient needs and resources. However, the concepts of moral stress and distress vary and there is unclarity regarding the definitions used in the literature. This study aims to (...) map and analyze the descriptions used by HCWs regarding morally challenging situations (moral stress) and refine a definition through conceptual analysis. Methods Qualitative data were collected in a survey of 16,044 Swedish HCWs who attended a COVID-19 online course in autumn 2020. In total, 643 free-text answers with descriptions of moral stress were analyzed through content analysis. Results Three themes emerged from the content analysis (1) “Seeing, but being prevented to act; feeling insufficient/inadequate and constrained in the profession,” (2) “Someone or something hindered me; organizational structures as an obstacle,” and (3) “The pandemic hindered us; pandemic-related obstacles.” The three themes correspond to the main theme, “Being prevented from providing good care.” Discussion The main theme describes moral stress as various obstacles to providing good care to patients in need and acting upon empathic ability within the professional role. The themes are discussed in relation to established definitions of moral stress and are assessed through conceptual analysis. A definition of moral stress was refined, based on one of the established definitions. Conclusions On the basis of the study results and conceptual analysis, it is argued that the presented definition fulfils certain conditions of adequacy. It is essential to frame the concept of moral stress, which has been defined in different ways in different disciplines, in order to know what we are talking about and move forward in developing prevention measures for the negative outcomes of this phenomenon. (shrink)