Midpoint and endpoint impact categories (or metrics, but not all impact categories can be converted into metrics) are the concepts in the area of life-cycle assessment (LCA) studies.
Midpoint impacts are precursors to some endpoint impacts that we care about. For example, greenhouse gas emissions are a midpoint impact that leads to climate change and, ultimately, to human loss of life due to food shortage, flooding, wildfires, and other catastrophic events, as well as loss of animal life and entire animal and plant species (endpoint impacts). For another example, ozone depletion is midpoint impact and increased cancer rates (because less UV radiation from the sun is blocked) is an endpoint impact.
Some impacts could be considered both midpoint and endpoint, e. g. loss of ecosystems is both an endpoint impact in itself, but also decreases the quality of life, prosperity, or even leads to famine among people who lived off these ecosystems.
Part of Life-Cycle Assessment Considerations for Batteries and Battery Materials