Aims. This paper (i) reviews the language used to describe and manage those patient practices that fall outside standard medical models of opioid treatment (for pain and opioid dependence), and (ii) proposes a consistent terminology that can be applied across multiple healthcare settings. Method. Peer-reviewed and grey literature documenting empirical studies of (non-)adherence with opioid treatment, proposed definitions or other potentially important aspects of terminology were included in this review. Results. There are international inconsistencies in the terminology used to describe the unintended consequences of opioid treatment, and the terms used often lack specificity. The terms 'hazardous use', 'extramedical use', 'opioid dependence', 'diversion', 'non-adherence' and 'aberrant behaviours' are defined. We advocate for consistent application of these terms in the context of opioid treatment, and propose that care is taken to describe individual practices and intentions. Conclusions. The increasing global attention on the use and diversion of pharmaceutical opioids warrants a discussion of current terms and definitions. Exaggerated concerns regarding 'addiction potential' may result in restrictions in the supply of opioids and the under-treatment of legitimate medical conditions. Researchers, clinicians, policy-makers and patients need to ensure greater care is given to terminology, including detailed descriptions of patient practices, the context in which they occur and severity of associated harm.[Larance B, Degenhardt L, Lintzeris N, Winstock A, Mattick R. Definitions related to the use of pharmaceutical opioids: Extramedical use, diversion, non-adherence and aberrant medication-related behaviours. Drug Alcohol Rev 2011;30:236-245] �� 2011 Australasian Professional Society on Alcohol and other Drugs.