Well-informed choices about how to improve health outcomes depend on access to reliable information, including research evidence. Many people (the public, health professionals and policymakers) have problems understanding some of the terminology used in describing evaluations of treatments. In this article, we describe the development of a glossary to provide plain language explanations of commonly used terms. We identified terms for inclusion in the glossary by screening multiple sources, including 14 other glossaries. Using an iterative process that included feedback and user testing, we designed an online glossary that is freely available at http://getitglossary.org. The glossary includes 242 commonly used terms. For each entry there is a suggested preferred term (plain language term), a short definition that can be accessed as a 'pop-up' within text, synonyms, a longer explanation and links to related terms. We also flag 11 terms that we advise against using because they are ambiguous or misinterpreted, with explanations of why we advise against using them. The glossary can be incorporated into documents, providing support for evidence-informed health-care decisions, and embedded in other websites serving a wide variety of functions.