INTRODUCTION: Children with medical complexity (CMC) are a subpopulation of children with chronic disease who live with extreme medical and social impairment. Identification of these children remains vague and controversial but in 2011, Cohen et al. formulated a conceptual definition composed of four required domains: Family Needs, Healthcare Use, Functional Limitation, and Diagnostic Conditions. Unfortunately, there has not yet been agreement on the specific criteria to fulfill each of the four domains. A consistent definition is an important requirement for clinicians, researchers, and policy makers to describe this growing population, understand their needs, develop evidenced-based policy and research management strategies and models of care. OBJECTIVE: The first objective of this research was to search the literature for variables associated with CMC and that would theoretically fulfill one of the domains. The second objective was to convert these variables into items which had the potential to fulfill their respective Cohen et al. 2011 domain, as determined by an expert panel in a Delphi study. The overall goal was to create a standardized definitional tool for CMC. METHODS: A scoping review of the CMC literature was completed and variables that could theoretically fulfill one of the four domains were extracted. These variables were converted to items which were collated into a survey delivered to an expert panel. Researchers and clinicians with CMC expertise were gathered from a variety of professional societies to undergo a four-iteration Delphi procedure. RESULTS: From the literature, 1411 studies were considered and 132 of these informed 55 items which were presented to the expert panel. Eighty-two experts were recruited from literature authorship and professional societies to complete the Delphi study. With an average response rate across iterations of 70%, these experts reached consensus for inclusion on 48 items and reached consensus for exclusion of 1 item, for a total consensus rate of 89%. CONCLUSION: The consensus items were collated into a definitional tool for CMC with 25 items presented to fulfill the Family Needs domain, 10 items in the Healthcare Use domain, 2 items in the Functional Limitations domain and 1 item in the Diagnostic Conditions domain. These results represent the first step to create a standardized definition of CMC.