Rapid reviews were first mentioned in the literature in 1997, when Best et al. described the rapid health technology assessment program in the south and west regions of England but did not provide a formal definition. More recently, the only consensus around a rapid review definition is that a formal definition does not exist. The primary aim of this work is to create a repository of existing definitions and to identify key themes, which may help the knowledge synthesis community in defining rapid review products. A systematic scoping review was performed to identify definitions used in journal-published rapid reviews written in English between 2017 and January 2019. We searched Medline, Embase Classic + Embase, PsycINFO, ERIC, Cochrane Library, CINAHL, and Web of Science on December 21, 2018. Two reviewers performed study selection and data extraction using a priori–defined methods published in a protocol. Definitions from rapid review methods articles (published from 1997 onward) identified in another scoping review were added to the results, and all definitions were thematically analyzed using NVivo. A quantitative analysis was also performed around studies cited. Definitions from 216 rapid reviews and 90 rapid review methods articles were included in the thematic analysis. Eight key themes were identified: accelerated/rapid process or approach, variation in methods shortcuts, focus/depth/breadth of scope, compare and contrast to a full traditional systematic review, stakeholder rationale, resource efficiency rationale, systematic approach, bias/limitations. Secondary referencing was a common occurrence. Thematic analysis performed in this systematic scoping review has allowed for the creation of a suggested definition for rapid reviews that can be used to inform the systematic review community.