In this study, we introduce a novel heatwave characterization metric: the Moisture ELevated Temperature (MELT) index. MELT integrates anomalies in temperature and relative humidity to quantify moist heatwaves and more accurately assess physiological heat stress. Traditional heatwave metrics predominantly rely on temperature alone, often underestimating the compounded effects of humidity on human health and thermoregulation. To address this gap, the MELT index offers improved accuracy for public health risk assessment and response strategies. To validate MELT's effectiveness and versatility, we applied it to analyze three significant, record-breaking heatwave events from recent decades: the 2021 Pacific Northwest (PNW), 2016 South Korea, and 2019 Western Europe heatwaves. Our analysis demonstrates that MELT clearly distinguishes between humid and dry heatwave conditions, accurately identifying the moisture characteristics specific to each region. Specifically, the PNW and South Korea events exhibited notably higher humidity levels, influenced by atmospheric rivers and increased convective activities, respectively. Conversely, the Western Europe heatwave was characterized by drier conditions resulting from Saharan dry-air intrusions. MELT's reliance on widely accessible datasets of temperature and humidity ensures its global applicability and consistency, addressing limitations inherent in temperature-only indices. Furthermore, its flexible use of climatological percentile thresholds allows adaptation to varying climates and future scenarios. Given anticipated increases in heatwave frequency and intensity due to climate change, MELT provides a critical tool for evaluating emerging risks, informing climate adaptation policies, and guiding targeted mitigation measures.