Background <p>Although guidelines for controlled trials have proposed a definition for a migraine day, substantial variability remains across clinical trials, highlighting the need for a universally adopted definition.</p> Methods <p>Through three survey rounds using an eDelphi method, a panel of 18 headache experts evaluated clinical scenarios to achieve consensus on a standardized definition of a migraine day. The first two rounds provided clear-cut cases with the question whether a clinical scenario should be labelled as a migraine day and the third round proposed a migraine day definition. Consensus was predefined at 70% agreement.</p> Results <p>A consensus was reached on a proposed definition of a migraine day across three distinct situations: (1) without acute treatment, (2) with acute treatment, and (3) in the presence of aura. If untreated, a headache day can be labelled as a (probable) migraine day when the following criteria are met: headache lasting ≥ 4&#xa0;h having at least two out of four headache characteristics, regardless of the presence of any associated symptom or at least one out of four headache characteristics combined with one associated symptom. If treated, a headache day can be labelled as a migraine day when the following criterion is met: lasting ≥ 30&#xa0;min, regardless of drug (class) taken, the presence of any headache characteristic or associated symptom. In the presence of aura, a migraine day can be defined as any calendar day on which the following criterion is met: an aura, fulfilling the ICHD-3 criteria for migraine with aura, is experienced regardless of the presence of headache.</p> Conclusions <p>We suggest the Classification Committee considers this eDelphi survey based consensus definition as a possible definition both for clinical trials and clinical care in the new edition of the International Classification of Headache Disorders.</p> Clinical trial number <p>Not applicable.</p>