Irritability is a symptom of 15 disorders in the DSM-5 and is included in Mood Disorders, Addictive Disorders, Personality Disorders, and more (American Psychiatric Association, 2013). However, the term irritability is defined and measured inconsistently in the scholarly literature. In this article, we reviewed the scholarly definitions of irritability and the item content of irritability measures. Components of definitions and items measuring irritability were divided into three categories: a) causes, b) experience, and c) consequences. We also reviewed potential causes and biomarkers of irritability. We found much overlap between definitions and measures of irritability and related constructs such as anger and aggression. Consequently, the validity of research on irritability needs to be questioned including the role of irritability in psychopathology and the presence of irritability as a symptom in any disorder. Research on irritability's role in behavioral disorders needs to be repeated after more well defined measures are developed. We proposed a more precise definition of irritability that clearly differentiates it from related constructs. Suggested items for measuring irritability are also provided. (C) 2017 Elsevier Ltd. All rights reserved.