This is the fifth paper in the evidence-based medicine glossary series. In this paper, we mainly introduce the systematic error（bias）and random error in medical research, review the definition and classification of bias made by different institutions and individuals. We also identify and categorize more than 10 types of bias in systematic review, which are considered the best evidence in evidence-based medicine. Additionally, we introduce some methods to reduce and eliminate bias, explaine six subject headings related to bias in MeSH, and introduce a new glossary — uncertainty in metrology.