Achieving global sustainability objectives such as the UN Sustainable Development Goals or Aichi Targets, including remaining within planetary boundaries, necessitates proactively avoiding a proportion of the environmental impacts otherwise expected to result from economic development. Quantifying these “avoided” impacts is important for monitoring progress toward meeting sustainability objectives, but doing so in a consistent way is fraught with difficulty. Using the mitigation of biodiversity impacts by development projects as an example, we explored the challenges of defining and measuring impact avoidance. Avoidance can be defined as either action-based or outcome-based and classified by whether it is achieved through project cancellation, spatial avoidance, design-based avoidance, or temporal avoidance. We also examined what drives different types of project proponents to implement avoidance measures. To support empirical quantification of the contribution that avoidance makes toward conservation goals, we present a framework for structuring assessments of biodiversity impact avoidance. Our framework has widespread applicability in conservation science, policy, and practice, as well as relevance for broader policies that seek to avoid environmental and social impacts.