Effective, unambiguous communication is essential for efficient translation of promising scientific discoveries into approved medical products. Unclear definitions and inconsistent use of key terms can hinder the evaluation and interpretation of scientific evidence and may pose significant obstacles to medical product development programs. Lack of clarity and consistency is also problematic in other scientific areas where FDA oversees product safety (e.g., foods and tobacco) to promote public health interests. In the spring of 2015 the FDA-NIH Joint Leadership Council identified the harmonization of terms used in translational science and medical product development as a priority need, with a focus on terms related to study endpoints and biomarkers. Working together with the goals of improving communication, aligning expectations, and improving scientific understanding, the two agencies developed the BEST (Biomarkers, EndpointS, and other Tools) Resource. The first phase of BEST comprises a glossary that clarifies important definitions and describes some of the hierarchical relationships, connections, and dependencies among the terms it contains. The BEST glossary aims to capture distinctions between biomarkers and clinical assessments and to describe their distinct roles in biomedical research, clinical practice, medical product development, and in the regulation of products by FDA. Because the glossary is intended to be broadly applicable to multiple communities of users and stakeholders, its definitions address nuances of usage and interpretation for a wide variety of terms currently in use. Further, based on differing stakeholder needs, it has built in flexibility, when possible and appropriate, to accommodate those interests. NIH and FDA intend to use the definitions included in this glossary when communicating on topics related to its contents (e.g., biomarkers) to ensure a consistent use of the terms and therefore, a common understanding of the issues. The BEST glossary is meant to be a “living” resource that will be periodically updated with additional terms and clarifying information. We welcome feedback, including specific proposed edits with rationale, from all stakeholders, including the scientific and medical communities, patients, providers, industry, and regulators, so that as we refine and elaborate on these terms, they will remain relevant, thus fostering consistent usage and ultimately help to accelerate development and refinement of medical products which lead to improvements in health outcomes. Suggested revisions will be considered on a regular basis.