Objectives Chronic pancreatitis (CP) is a fibroinflammatory disease complicated by episodes of acute inflammation (acute on chronic pancreatitis (ACP)). This entity is common, variably defined and can reflect different pathological mechanisms that requires different interventions. The aim of this study is to conduct a systematic review of how ACP is described, defined and diagnosed in the literature. Methods A systematic search was conducted from January 1993 to June 2020. All articles that used a term to describe ACP in adults were reviewed and definitions and diagnostic criteria were sought. Results After reviewing 2271 abstracts, 848 articles included a term to describe ACP. The most common descriptions were 'acute on/in CP' (374), 'acute exacerbation of CP' (345) and 'flare(-up) of CP' (43). Among the 848 articles, 14 included a pragmatic definition of ACP, and only 2 papers stated diagnostic criteria. These covered both acute inflammation and acute exacerbation of chronic abdominal pain. Conclusion There is no universally accepted term, definition or diagnostic criteria for ACP. A consensus definition is needed to improve quality and comparability of future articles as well as clinical management.