Early diagnosis of persons infected with human immunodeficiency virus (HIV) through diagnostic testing and screening is a critical priority for individual and public health. Emergency departments (EDs) have an important role in this effort. As EDs gain experience in HIV testing, it is increasingly apparent that implementing testing is conceptually and operationally complex. A wide variety of HIV testing practice and research models have emerged, each reflecting adaptations to site-specific factors and the needs of local populations. The diversity and complexity inherent in nascent ED HIV testing practice and research are associated with the risk that findings will not be described according to a common lexicon. This article presents a comprehensive set of terms and definitions that can be used to describe ED-based HIV testing programs, developed by consensus opinion from the inaugural meeting of the National ED HIV Testing Consortium. These definitions are designed to facilitate discussion, increase comparability of future reports, and potentially accelerate wider implementation of ED HIV testing.