Persistent headache, daily from its onset which is clearly remembered. The pain lacks characteristic features, and may be migraine-like or tension-type-like, or have elements of both.
Diagnostic criteria:
A. Persistent headache fulfilling criteria B and C
B. Distinct and clearly-remembered onset, with pain becoming continuous and unremitting within 24 hr
C. Present for >3 months
Source
Headache Classification Committee of the International Headache Society (IHS) (2013). The International Classification of Headache Disorders, 3rd edition (beta version). Cephalalgia 2013; 33: 9-807.