Background: While students possess procedural knowledge associated with engineering calculations and calculus, they often exhibit a disconnect between the conceptual meaning of mathematics and the problem being solved. This conceptual understanding of what mathematics "says" is defined by the theoretical framework of numeracy. Many engineering numeracy studies have been conducted, but they lacked a unifying theoretical framework, a common lexicon, or formal definitions to connect related numeracy topics.Purpose/Hypothesis: This study aims to define numeracy in the context of engineering mathematics and add to the broad theoretical framework of engineering numeracy. Formal skills, terms, and definitions are presented to generate further research. Design/Method: A Delphi study was conducted to identify which skills constitute engineering numeracy and define engineering numeracy skills. Also, a content analysis of 5466 articles published between 1970 and 2018 was conducted to validate the findings of the Delphi panel. Results: The Delphi study identified 32 skills and subskills, each validated and refined by the content analysis. The content analysis highlighted the need for more engineering numeracy studies. The difficulty and importance of learning different numeracy skills were also established.