Abstract The term “performance portability” has been informally used in computing to refer to a variety of notions which generally include: (1) the ability to run one application across multiple hardware platforms; and (2) achieving some notional level of performance on these platforms. However, there has been a noticeable lack of consensus on the precise meaning of the term, and authors’ conclusions regarding their success (or failure) to achieve performance portability have thus far been subjective. Comparing one approach to performance portability with another has generally been marked with vague claims and verbose, qualitative explanation of the comparison. This article presents a concise definition for performance portability and an associated metric that accurately capture the performance and portability of an application across different platforms. Through retroactive application of this metric to previous research and a review of numerous programming languages, frameworks and libraries, we devise and suggest tractable approaches to code specialization which can aid the community in developing highly performance-portable applications with minimal impact to productivity.