The aim of this Delphi survey is to develop common definitions for unlicensed and off-label drug use in children to be used for research and regulatory purposes. After a literature review on the current status of unlicensed/off-label definitions, a two-stage, web-based Delphi survey was conducted among experts in Europe. Their opinion on concerns, rules and scenarios regarding the unlicensed and off-label use of medicines were obtained. Results were then consulted with the European Medicines Agency (EMEA) before the final proposal was circulated to participants. Eighty-four experts were invited to participate (scientists, health professionals, pharmaceutical companies, regulatory agencies), 34 responded to the first round questionnaire and participated in subsequent rounds. Consensus was reached for the majority of questions. The lowest level of consensus reached was for questions related to a different formulation or if a drug was given although contraindicated. At the final step, 85% of the responding experts agreed on the proposed definition for off-label (use of a drug already covered by a Marketing Authorisation, in an unapproved way) and 80% on the definition for unlicensed (use of a drug not covered by a Marketing Authorisation as medicinal for human use), respectively. Results will facilitate the conduct of pharmacoepidemiological studies and allow comparison between different countries. The Delphi panel agreed that the definitions should be circulated within the scientific community and recommended to be adopted by relevant regulatory authorities.