Background: With the rapid development of evidence-based healthcare during the past two decades, evidence and evidence-based methods were not only used in the field of health care, but also applied to other non-medical fields. Since we are talking about evidence based decision-making we first need to see what we really mean by the word evidence and what connotations and extensions it contains. Objective: To find out whether the evidence has been defined and compare with the advantages and disadvantages of these definitions. Methods: Eight databases (to March 2010) including MEDLINE, EMBASE, SCI, CBM, CNKI, VIP, WANGFANG, Cochrane library and relevant websites were searched. The search terms were evidence-based medicine , evidence-based healthcare , concept , definition , and so on. Results: Seven definitions were included (see table). Conclusions: Only two definitions were based on systematic reviews. The other definitions can not fully reflect the concept of evidence because some are too wide or complicated and some are too narrow or simple, we recommend using the definition that evidence is the information from systematic reviews. The definition derives from the methods of systematic reviews and it is easy to understand and dissemination.