Heatwave effects on human health and wellbeing is a great public health concern, especially in the context of climate change. However, no universally consistent heatwave definition is available. A systematic review and meta-analysis was conducted to assess the heatwave definitions used in the literature published up to 1st April 2015 by searching five databases (PubMed, ProQuest, ScienceDirect, Scopus, and Web of Science). Random-effects models were used to pool the effects of heatwave on total and cardiorespiratory mortality by different heatwave definitions. Existing evidence suggests a significant impact of heatwave on mortality, but the magnitude of the effect estimates varies under different heatwave definitions. Heatwave-related mortality risks increased by 4% (using "mean temperatures ≥ 95th percentile for ≥ 2 days" as a heatwave definition), 3% (mean temperatures ≥ 98th percentile for ≥ 2 days), 7% (mean temperatures ≥. 99th percentile for ≥ 2 days) and 16% (mean temperatures ≥ 97th percentile for ≥ 5 days). Heatwave intensity plays a relatively more important role than duration in determining heatwave-related deaths. Heatwaves significantly increase mortality across the globe, but the effect estimates vary with the definition of heatwaves. City- or region-specific heat health early warning systems based on identified local heatwave definitions may be optimal for protecting and preventing people from the adverse impacts of future heatwaves.