Background Intraoperative hypotension (IOH) during non-cardiac surgery is common and associated with major adverse kidney, neurological and cardiac events and even death. Given that IOH is a modifiable risk factor for the mitigation of postoperative complications, it is imperative to generate a precise definition for IOH to facilitate strategies for avoiding or treating its occurrence. Moreover, a universal and consensus definition of IOH may also facilitate the application of novel and emerging therapeutic interventions in treating IOH. We conducted a review to systematically record the reported definitions of intraoperative hypotension in adults undergoing non-cardiac surgery under general anaesthesia. Methods In accordance with Cochrane guidelines, we searched three online databases (OVID [Medline], Embase and Cochrane Library) for all studies published from 1 January 2000 to 6 September 2020. We evaluated the number of studies that reported the absolute or relative threshold values for defining blood pressure. Secondary aims included evaluation of the threshold values for defining IOH, the methodology for accounting for the severity of hypotension, whether the type of surgical procedure influenced the definition of IOH, and whether a study whose definition of IOH aligned with the Perioperative Quality Initiative-3 workgroup (POQI) consensus statement for defining was more likely to be associated with determining an adverse postoperative outcome. Results A total of 318 studies were included in the final qualitative synthesis. Most studies (n = 249; 78.3%) used an absolute threshold to define hypotension; 150 (60.5%) reported SBP, 117 (47.2%) reported MAP, and 12 (4.8%) reported diastolic blood pressure (DBP). 126 (39.6%) used a relative threshold to define hypotension. Of the included studies, 153 (48.1%) did not include any duration variable in their definition of hypotension. Among the selected 318 studies 148 (46.5%) studies defined IOH according to the POQI statement. When studies used a “relative blood pressure change” to define IOH, there was a weaker association in detecting adverse postoperative outcomes compared to studies who reported “absolute blood pressure change” (χ2(2) = 10.508, P = 0.005, Cramér’s V = 0.182). When studies used the POQI statement definition of hypotension or defined IOH by values higher than the POQI statement definition there were statistical differences observed between IOH and adverse postoperative outcomes (χ2(1) = 6.581, P = 0.037, Cramér’s V = 0.144). When both the duration of IOH or the numbers of hypotensive epochs were evaluated, we observed a significantly stronger relationship between the definition of IOH use the development of adverse postoperative outcomes. (χ2(1) = 4.860, P = 0.027, Cramér’s V = 0.124). Conclusions Most studies defined IOH by absolute or relative changes from baseline values. There are substantial inconsistencies in how IOH was reported. Further, definitions differed across different surgical specialities. Our findings further suggest that IOH should be defined using the absolute values stated in the POQI statement i.e., MAP < 60–70 mmHg or SBP < 100 mmHg. Finally, the number of hypotensive epochs or time-weighted duration of IOH should also be reported.