Recognizing and measuring diagnostic error can be challenging, which hinders efforts to study and improve diagnosis. This systematic review of 123 studies sought to characterize diagnostic uncertainty. Despite the lack of an explicit definition in any study, researchers identified diagnostic uncertainty as a clinician perception that affects diagnostic evaluation and changes over time. Strategies to measure diagnostic uncertainty included assessing clinician perceptions through survey or interview methods, examining the diagnostic evaluation through medical record review, or employing simulation with standardized cases or vignettes. The authors propose the following definition of diagnostic uncertainty: "subjective perception of an inability to provide an accurate explanation of the patient's health problem," paralleling the National Academy of Medicine's definition of diagnosis. A recent WebM&M commentary discussed how cognition influences diagnostic decision-making.